You forgot the PSU costs. Everyone always forgets the PSU cost. The S3 does not cost $305, it costs at least $350.
Only if you're setting up a single S3. If you doing any sort of farm, you're going to use server power supplies which cost maybe $10 per S3.
Got any examples? This seems unlikely to me.
The Dell 750w can be purchased for around that, a few cables and a large enough quantity would get that price.
Or you could purchase a breakout board adds a bit more to the PSU cost, its easier and sometimes looks nicer.