It can be as cheap or expensive as you want it to be
Suthrn gave you some pretty good ballpark numbers. Craigslist can be your friend if you're lucky. In my case, I found two 20 lb CO2 tanks for $80 total, and were each at least half full. Then I found a few kegs on ebay that came out to about $27 each including shipping. I splurged a bit and went for a three body regulator for about $100. Then you obviously have the misc. fittings, tubing, clamps. The only other major expense I had was the chest freezer which ran another $400 or so, but it will hold 6 or 7 kegs. Oh, and I forgot about the temp controller. I can't remember what that was, but I think it was another $50 or so.
For now, I'm not running any fancy tap system and just using picnic taps and have to lift the lid of the freezer to pull a pint. I spent a little too much on all the hardware so I have to wait a bit before getting permanent taps set up. But in your case, if you're already getting some taps for free that isn't much of an issue