1. Find a cheap AM3 motherboard. They can be found with 1 pci-e 16x slot and 2 x 1x slots for under $50 ( you will need 2 pci-e 1x to 16x risers )
2. AMD sempron single core CPU $30
3. 1 2GB stick of ddr-3 ram $20
4. Decent 750w Power supply $80
So lets call that $200 rounding up a little for tax. Now for the GPU's
6950's can be had for about $100 on ebay these days so...
3x 6950 = 1200 - 1500 Kh = $300
Bringing your total to $500
EDIT: oh ya add $5 for a flash drive and a copy of BAMT
Lets go off of 1.25Mh is $3.50 a day
Noow lets factor is a 10% drop in profitability every month
Your estimated ROI is about 7 months
Unless you are actively trading and decent with your trading returns your not going to make much
Or if you get real lucky and a legit new coin comes out that will actually take off and you can become an early adopter of (not just another crappy alt coin)