Let's take a system with peak GFX card output from two overclocked GTX 670 DCI TOPhttp://www.guru3d.com/article/asus-geforce-gtx-670-directcu-ii-top-review/8
Power Consumption with one card = 293 watts
2nd Card = 161 watts
Total = 454 watts
454 watts x 30 hours per week x 4.3 weeks per month x $0.10 per Kw-hr / 1000 watts per kw/hr = $5.86 per month bill
And that's full tilt all the time
With system at idle we are looking at 141 watts + 10 watts +10 watts = 161 watts or 35% of that or $2.00 a month
That only factors in the GPU being at full power though, not the CPU as well. It would still be <500W though, it seems, and BOTH being at 100% at the same time is VERY rare, I know (never happens unless stress testing both).
I was working in ~80% efficiency in there as well, though.
Still, I stand by my general usage wattage being <250W (probably closer to 200W), just not 161W either, since it won't be at true