So if I put together a few numbers:
device speed cost hash/s/$
3x780ti 7000000 3*$700 3333
1xi7 420000 $280 1500
That gives an efficiency advantage of about 2.2 (I'm doing this from memory - I think the actual hashrate for the GPU rig was like 7.2, leading to an advantage of 2.3).
Take into account power consumption. Around 220 W for 780ti (at least for some workloads I managed to google).