Okay finished my spreadsheet,
Made two systems, a low cost to entry system and an optimal system
cost
G-B B75M-D3V 62
Intel G1610 44
7970 409
7970 409
GS800 127
memory 16
hdd use a spare
total cost 1067
MH/s 1300
power usage 750
$ per MH/s 1.2183692596
G-B B75M-D3V 62
Intel G1610 44
7870 229
7870 229
GS600 95
memory 16
hdd use a spare
total cost 675
MH/s 800
power usage 562.5
MH/s per $ 1.1851851852
video card analysis
Video card MH/s TDP cost $ per MH/s watt per MH/s
7750 125 55 89 1.404494382 2.2727272727
7770 200 80 109 1.8348623853 2.5
7850 300 130 209 1.4354066986 2.3076923077
7870 400 175 229 1.7467248908 2.2857142857
7950 500 200 318 1.572327044 2.5
7970 650 250 409 1.5892420538 2.6
7990 1200 555 ? #VALUE! 2.1621621622
what do you think?
I assumed that the cpu doesn't matter so you should get the cheapest / lowest power ones (this might not be true)
I assumed 100w system power draw before vid cards and that the PSU would be 80plus and you would try and run it at 80% for max efficiency
all prices are from MSY
I've just chosen gigabyte mobos because I like them and i didn't want to make it all too complicated.
I couldn't find any mobos on there that you should put 3GPU;s in ( i assumed a 4x slot was not enough)