How do I fix this leaking tub faucet?
A few years ago I replaced the "guts" of my shower's faucet. It was about 10 years old and leaking, never quite shut all the way off. It is not hard at all. Just take off the knobs/face plates and there is usually a special tool that come with the replacement cartridge that allows you to yank the old one out. I probably yanked the old cartridge out with my own pliers but I can't remember. Point is you can do this for far less than $500 and its as easy as sliding the old cartridge out and the new one in. Since yours is 20 years old your biggest problem may be finding the make/model of the specific unit so that you can get the correct replacement cartridges for it.
A "rebuild" kit sounds like it comes with garbage you probably aren't going to need or can buy separately. A replacement cartridge is generally $23 and comes with all the o-rings you need.