if we are talking about your 2P Opteron rig should be somewhere around 350W, also depending what graphics card you use. The TDP of Opteron 6128 is 115W so this is max wattage the CPU will consume, which i guess is the case when folding. Let's say it will consume 350W, keeping it on for 1 hour means it consumes 0.35kWh, multiplied by 24 is 8.4 and by 7 is 58.8. So it is 58.8 kwh/week,. multiply that with the cost of electricity in your province and you get the price of the consumed current fro the PSU. Of course you will have to add the percentage of efficiency of the PSU that is lost in conversion. let's say at 350W your PSU is 88% efficient you will have to add that to the price so let's say 10% more it will be around 63 kWh/week. If you want to minimize the costs it will be logic to invvest into a PSU that will have a very high efficiency, 80 Plus Gold or Platinum and the PSU will have to be chosen as the required load to be around 60% of its nominal power. For example you need 350W at full load, make it a 600W PSU. It happens that I use a 650W Antec Earth Watts Green on my Opteron 2P folder platform and at 400W load efficiency is a bit higher than 88%. Review here

Antec Earthwatts EA 650W green Review | KitGuru
Attached you will find a document from Hydro Quebec with a comparison of electricity prices in North America as of 2011, residential consumers is at page 9.

This morning I did some calculations of GPU vs SMP folding costs based on the prices of a used GTX 570 at 200$ and a system with a core i7 2600k which came out at 500$, considering you find the CPU used at 275$. I considered the cycle as being 2 months, at least Hydro Quebec is doing the bill at 2 months, cost of electricity 7c/kWh, PSU efficiency 90% and folding done 24/7. The difference in cost for the i7 system will be offset after 23 cycles that means 46 months, almost 4 years. So you will have to keep your system 4 years just to cover the difference you paid in components versus a powerful graphics card. But if we consider the fact that the difference in PPD in favor of the CPU versus GPU would be 10k PPD (as the situation is today, considering folding under Windows) it would also mean that the CPU would give a 14 million points advantage.

electricity cost of the components only, assuming that while folding they get to their max specified power draw :

gtx 480 289W = 3.75$/week = 30.00$/cycle

gtx 570 220W = 2.83$/week = 22.64$/cycle

i7 2600k 95W = 1.22$/week = 09.76$/cycle

i7 3770k 77W = 0.99$/week = 07.96$/cycle

GTX 570 = 200$

i7 2600k 275$ + mobo 150+ RAM 30$ + HSF 45$ = 500$