... above
You can boot off of a live Linux USB if needed, and RAM is cheap if you buy it used if you want to skimp on your rig and get a cheaper build. PSU and Mobo is what you'll be looking for. CPU doesn't really matter that much for the most part; a 3$ C2D will work most of the time.
True but assuming you are getting a nice PSU to power up say 6 470's, if you go with nice ones such as EVGA 1300's or other close to it that add's up and will take a bite out of budget. Then a motherboard that does 5 or 6 GPU's again add's up. The cpu/ram might be able to get cheap but those and riser's still can add up on a build.
So I agree some part's are cheap compared to say an asic cost. But overall they do add up and cost money which add more money you need to ROI. My main point was GPU's are not only cost.