Depends on what you want. I bought a fridge on Craigslist for $30.00, 3 corny kegs and 5lbs CO2 bottle on Craigslist for 100.00 and a fridge conversion kit on Micromatic.com for 65.00.
I built my own, probably less than 200 in it all, but I used a lot of old stuff that I had laying around. Hence the name of it by the way, Franken-Keezer. Here is the link to the pics that I have, https://cdn.homebrewtalk.com/gallery/showphoto.php?photo=33217&nojs=1 I had a lot of fun building it, SWMBO on the other not so much. Good luck on your build, and remember as long as you like it, to hell with everyone else...