Yeah, I don't think you will see much of an increase if you are just running folding on a PC you have on anyway. If you are running a dedicated machine then the result might be different.
It easy to figure out, just find out how many watts you rig uses with the gpu at full load and calculate the running costs by multiplying that times the charge per kilowatt hour.
The difference between PSU's is amazing. Before I had a OCZ 700W and idle my UPS system said it was taking about 350W of power with 1 monitor. I now have the PC P&C 750W with 2 monitors and at load it uses 320W. I do have only a dual core now but there is still is an amazing difference.