So $100 per PCIe slot.
$50 Sempron 145
$10 2gb ram
$174 MSI 890FXA-GD70(I bought a number of them @ $125 open box)
$200-$250 your flavor of 1200w PSU
$25 5 PCIe extender cables
$5 4gb USB stick + BAMT/Ubuntu
Comes out to ~80/PCIe slot but you have 1 machine to manage instead of 6. The resale of hardware will also be higher.
With only 7 computers I wish I could combine them all in to one machine to manage. I couldn't imagine doing this with ~35 machines.
I was thinking 4 cards per rig until I thought... what happens when 1 board or PSU goes up in smoke? 4 cards are down.
If the op was thinking of these... http://www.newegg.com/Product/Product.aspx?Item=N82E16813157209
However, as others have said, I wouldn't necessarily go for the cheapest PSU. Cut the 16gb down to 4, get a used 24 10/100 switch for $10 or $20 bucks.
The other thing is your power inefficiency goes down when you have 1 underloaded PSU per board.