For gaming the GPU folding has to be stopped. GPUs don't have process management and priority like CPUs, so when you run two compute intensive things on them (Game and Folding) you get a train wreck.
I would really just recommend using advanced control to pause folding while you game and turn it back on afterward. If you leave advanced control running on your desktop it should just be a single click off and a single click back on.
The power load over idle will be significant. 970's are some of the most efficient GPUs out there for folding, so while you would be using a good bit of power you would be getting a lot of work done.
As for cost of the power, what it the rate per kWh in your area? Each of those GPUs at full load is over 100W (150W?), so total system is 400W perhaps (best way is to measure with a power meter like a kill-a-watt). With that you can figure out your cost per month that running the GPU adds.