I built my 3 tier system about 6 months ago, and did it for pretty close to $200. Here are most of the parts I used.
angle iron and bolts to build the frame: ~$100
2 kegs(1 was mine, another donated from a friend): free
10 gallon igloo water cooler: $40 from home depot
2 propane burners from the side of bbq's: one free from uncle's old bbq, another for $10 off of craigslist
propane tank came with the craiglist bbq, and some hose clamps and tubing are cheap to connect
brass nipples, shutoff valves, and through hull fittings: ~$70 at west marine
I think that is all the major parts that i needed, hopefully that helps some.