It depends on the type of problem you're trying to fix. Some parts cost more than others. In some cases it won't make sense to fix but rather to get a new camera.
The prices are normally between 50-200$ for a normal camera. If you want one with some big zoom, you'll see some for 200-500$ Higher than 500$ should be DSLR. (it goes up to 10,000$)---------- EDIT...
It depends on what type of underwater digital camera you are wanting to purchase and how fancy of one you want to get but the range can be as low as 90 dollars and as expensive as 300 dollars.
The long answer depends on what's wrong with the lens, but the short answer is easier. If you are not a trained technician with the proper tools, you don't.
An exception is "lens errors". Lens...