You need:
1. At least 1 keg - figure $30-$70 used
2. Co2 tank - $50?
3. Regulator - $60
4. Somewhere to keep the keg cold - see below.
5. Gas line, beverage line, picnic tap $15-20 or so.
For example, I picked up from kegconnection around the holidays a kit with a keg, 4# Co2 tank, two gauge regulator, gas line with quick disconnect, bev line with quick disconnect and picnic tap for under $120. For #4, you can buy a kegalator, or convert an old refrigerator, or create a keezer from a freezer (old or new). I have both a converted refrigerator and a keezer. In the long run I would recommend the keezer route...all that is required to start is a used freezer, as long as the refrigerant lines aren't in the shelves along with an STC-1000 (aka "ebay digital temperature controller") which you use to externally control the freezer operation so that it maintains temperature in a beer range instead of freezing. The freezer will cost you whatever (used), the STC-1000 costs about $23-$24 plus you will need to spend $10 or so to put it in a project box and wire it up. If you go the used refrigerator route, you avoid the need to control the temperature externally. Hopefully I didn't over-complicate this, please feel free to ask questions.
Once you go the kegging route, you will wonder why you ever bottled!
