boju, that's not the whole picture, and compared to a 780 GTX the TDP difference 85w
So in SLI, that's 170w difference.
So by your math that saves $17 a month. But that's not all. That's also 170 watts less heat getting pumped into the room. Sure that doesn't mean much to you if you're living in canada, and gaming in the winter where you're struggling to keep the room warm enough to not freeze to death. But move that same rig into a place like Florida or Texas, in the summer, where you gotta blast the AC constantly just to not be drentched in sweat, and you would be very thankful for that much less heat getting pumped into your room.
So less power on the system, less power spent on AC, more comfortable experience while gaming, and not to mention lower temps can help with better overclocks.
Which the overclock factor imho is way more important than saving money on a power bill.