Depends on what kind of system you are looking for. I think for a barebones (tank, regulator, cornie keg, gas-line and beverage line (picnic tap) you can get systems like this from Northern Brewer and I think Austin for ~150 or so dollars.
I got a 20lb Co2 tank from the coke distributer for 40 (deposit) and 18 (gas). Now I just pay 18 for an exchange. It's not as pretty but a one time pay out of 40 dollars for a 20lb tank is a pretty good deal.