Let's start by looking at what things in the bathroom typically need to be redone, and then at what options you can use to keep your cost minimal, or to skyrocket them.

The size:

If you want a larger bathroom, you won't be able to do it for under $500. Resize only if absolutely necessary to accommodate your needs. This is the single most expensive aspect of the bathroom renovation. So, learn to live with the size you have, or drastically change the budget amount for your bathroom remodel.

Tub and shower area:

Because of the constant use tubs and showers get, they often need to be redone simply because they look dingy, or have gotten outdated, chipped, etc. In some cases people want to change them out because they want a bigger tub, a different enclosure, or jets, etc. If your bathtub needs to be fixed or replaced, there is a good chance you won't get a bathroom remodel complete for under $500. Even though tubs do not always cost very much, when you add in the cost of the contractor, removal, demolition, etc. the costs really start to add up. So, if looking to complete your remodel for under $500, salvage your tub or shower area. If you are using a $1500 budget, this is something you can replace, and with a larger budget, you probably will want to replace this.

To best salvage your tub or shower area, do the following:

First, clean your tub as best you can with a commercial grade bathtub cleaner. Consider using a cleanser that removes calcium, lime, and rust. It is amazing what a high grade cleaner can do for your tub or shower area, especially if it has not been cleaned as regularly as it should have. Make sure when you are cleaning that you have plenty of ventilation so that you do not make yourself sick.

If cleaning is not enough to restore the tub or shower area to a look that you are satisfied with, then consider raising your budget. Instead of replacing your tub, you can have it refinished, or have a liner installed over it. These can make a big difference aesthetically. However, if you want to keep the cost low, you have to really shop around for the right contractor.

Toilet:

Toilets are fairly inexpensive to purchase, and a new toilet can make a big difference in a bathroom, especially if you are replacing an outdated toilet with a new toilet. You will save money in the long run doing this because you will use less water. Installing a toilet is a common do-it-yourself job, so if you are serious about saving money, remove your old toilet, and replace it yourself. It may take you a few hours, but it is not that hard, and can save you money. If you decide you really want a contractor to do it, know that because it is an easy process they should not be charging you high labor costs. It should not cost you over $100 to have it installed. Just remember, that is 20% of your budget if your budget is $500, so once again, with a $500 or $1500 budget, it is best to do that yourself.

Walls:

The walls in a bathroom are often a part of why bathrooms need to be remodeled in the first place. There is a lot of water in bathrooms, and drywall often must be completely replaced in bathroom renovations due to the high moisture content. To keep your cost low, talk to your contractor about only replacing the pieces that are damaged, and not anything else. If you do not have damage, then walls are easy, a coat of paint that can withstand moisture is about all you need.

Cabinetry and vanity:

The vanity area of your bathroom can be really inexpensive to remodel, or really expensive. If you choose to replace an existing vanity with one that is a different size, the cost will jump. If you choose a high end vanity, again the cost jumps. So, to keep your cabinetry and vanity portion of your budget in check, consider buying from a second hand store. In many cases people pull out perfectly good vanities during a remodel in order to put something more stylish or to their taste in. You can purchase these at low price, and with a little cleaning, maybe some paint, they can be awesome. However, cheaper still is to simply refinish existing vanities, or paint.

Counter tops:

Counter tops are high dollar to replace, even if the actual material is cheap, the labor is not. So consider replacing the entire vanity area with one that is prefabricated with the counter top on it. Counter top options are immense. You can get marble, granite, solid surface, laminate, tile, etc. The cost of replacing your counter will depend on the surface area and the material of choice. To keep costs low, learn to live with what you already have, because it will be next to impossible to replace a counter top with only a $500 budget to do the whole bathroom. If you really need to replace the counter top, consider a new laminate top, over the existing counter structure, or look for remnant pieces of higher end materials.

Plumbing fixtures:

Plumbing fixtures means the shower head, the faucets, handles, etc. Once again the costs go from next to nothing, to extremely expensive. For example, if you were to walk into a Home Depot, you could pick up a shower head that works perfectly well for $6. Or, you could spend close to $300 on a shower head. The same is true for faucets, handles, etc. So, to replace your whole faucet system is fine, and can be done whether you have a $500 budget or a $15,000 budget, as long as you choose the right materials.

Sinks:

Sinks are a necessary part of a bathroom. There are tons of options for sinks, you can get under counter mounted sinks, you can get drop in, you can get the kind that sit on top of the counter. With each of these, and the counter they come with, is a different cost. To keep costs low, consider cleaning and relining your existing sink, or living with it as it is. For a medium cost remodel, consider replacing the whole vanity area, including sink with a one piece fixture, meaning the cabinet, counter, and sink all come put together for one price. With a larger budget, choose what you like!

Flooring:

Like your walls, in some cases floors, and subfloors need to be replaced because of water damage, mold, or high moisture content. The material you choose for your flooring with greatly impact the cost. A stick on linoleum for example can be laid yourself and costs as little as $0.49 a square foot. Travertine on the other hand costs far more, and you would need a contractor to lay it. So, keep the material differences, and the costs of labor in mind when considering your budget.

Electrical:

Generally if you are doing anything with electrical in a bathroom remodel, it is updating it because your system does not meet the current electrical code. However, this can be costly. So, if it is functional, your local permitting office may be able to tell you if your system can be grandfathered in, or if you can do a little tweaking to get it there, such as to replace non-grounded outlets with GFCI (ground-fault current interrupter) outlets. The cost to do this is going to depend on the electrician you use, so do some homework to find a good, fair, and appropriately priced electrician.

Plumbing:

Usually the only time you really worry about re-plumbing is if the sink, tub, and shower are being moved. This is impossible for a budget less than $1500. Reconsider plans you have for moving these items. You may find that you can live with your current layout, and can save a lot of money. If you need to re-plum, this means removing sub-floors, drywall, etc. so the cost is going to be very, very high!

Ok, so now that we know what things need to be considered in a bathroom remodel, let's take a look at what you can do specifically within the given budgets.

Bathroom remodel for less than $500:

With this kind of remodel you are doing basic aesthetics, and not much more. Let's say you have an average bathroom size with one sink, a toilet, tub/shower, and about 25 square feet of flooring.

Size: Leave the same.
Tub: Clean well. Estimated cost for cleaning supplies $25.
Toilet: Replace with an American Standard toilet from the Home Depot, estimated cost for all the necessary parts $125. Replace yourself.
Walls: Paint with a fresh coat. Paint cost for a gallon $25, rollers $5, tape, brushes, etc. $10
Cabinetry: Clean well. Replace hardware for $30.
Counter tops: Clean well.
Plumbing fixtures: Replace shower head for $30, labor at $50. Replace faucet for $45, labor $50.
Sinks: Clean well.
Flooring: Lay new linoleum at $0.49 a sq/ft, buy 50 square feet, for cost of $30.
Electrical: Leave.
Plumbing: Leave.

Total: If you do everything yourself except replacing your plumbing fixtures your total cost will be an estimated $425. It may be more or less depending on labor costs. This is like giving your bathroom a facelift, rather than a true remodel.

Bathroom remodel for less than $1500:

Let's say you have an average bathroom size with one sink, a toilet, tub/shower, and about 25 square feet of flooring.

Size: Leave the same
Tub: Clean well. Estimated cost for cleaning supplies $25.
Toilet: Replace with a new toilet that costs less than $250 total. Put in yourself.
Walls: Paint, $40
Cabinetry: Replace with pre-fabricated vanity that is less than $400. Labor $75
Counter tops: Included above.
Plumbing fixtures: Purchase fixtures that cost less than $100. Labor $50.
Sinks: Included in vanity.
Flooring: Choose a tile flooring that costs $3.50 a sq. ft., buy 30 sq. ft. at $115. Grout at $25. Installation at $200.
Electrical: Leave.
Plumbing: Leave.
Total: The total from above comes out to $1255. You get a new look on a lot. Of course, this can be adjusted, if the tub needs refinishing, go with a less expensive flooring option, and less expensive toilet. If your plumber's labor cost is more, there is room for that.

Bathroom remodel for $5000+

Let's say you have an average bathroom size with one sink, a toilet, tub/shower, and about 25 square feet of flooring.

Size: Leave the same.
Tub: Replace. Tub cost $500. Removal, etc. $250. Installation $300
Toilet: Replace. Toilet cost $250. Installation $100.
Walls: Replace drywall. Repaint. Cost $1500.
Cabinetry: Replace. Cost $1000.
Counter tops: Replace. Cost $750 with labor.
Plumbing fixtures: Replace. Cost $350 with labor.
Sinks: Replace.
Flooring: Replace
Electrical: Update.
Plumbing: Leave.

Total: It really just depends on what you choose, and how much you pay your contractors.