You can spend as much or as little as you want.
I got my keg setup from my LHBS for $200 (included 1 corny, 10# co2 tank, dual regulator, gas/liquid tubing, quick disconnects and they even threw in some hose clamps).
For a while I kegged and just stuck it in a fridge and used a party tap.
When I put my keezer together, I found a 5 cu.ft. Chest freezer on CL for $40 and I ordered a dual faucet tower for $150 (I think).
So, basically, you should be able to get what you need for $400