Seems like that is going to cost more than $10 per S3.
which cost maybe $10 per S3.
If you don't want the fancy board, and use just bare cables and something to hold them on the PSU you could find Dell 750w's for around $10 with a few dollars more to buy the cables.
you are forgetting the time and work required to modify the server psu - could take half an hour to cut cables and solder them on. Atop that, server gear may have lower efficiency and lifespan compared with an 80+ gold ATX supply, and can be way louder (if that matters in the setup)
I personally switched all my DPS800 stuff over to 1200W GOLD or better supplies when i found some on the used market at good prices