A quick look at Kegconnection (great place to buy keg stuff) for a bare bones setup puts a 4 keg system at a little under $400. That's with the 4-keg basic kit with picnic taps, 4 pin lock kegs, and a 5 lb CO2 tank. If you want to move up to fixed taps rather than picnic taps, you'll need an additional 4 shanks and faucets. Cheap standard faucets run about $15, while Perlick faucets run $40 for SS and $23 for chrome (highly recommend the Perlicks, whether you go SS or chrome). Shanks run another $13-20 a piece depending on chrome or stainless and the length. The kit I linked to before runs all 4 kegs at the same psi, so if you want different carb levels you'll have to upgrade the regulator, and might want to get a bigger CO2 tank depending on how close and cheap the co2 refill places around you are.
Edit: Forgot, these are for pin-lock kegs, which are a little shorter and fatter (but ~$10-20 cheaper) than ball-lock kegs. Double check your fridge space before getting them to make sure they'll fit.