I used a Kill-A-Watt meter to evaluate this very question. (
http://www.p3international.com/products/special/P4400/P4400-CE.html)
My 321 has two Seagate 1.5TB drives in RAID-1. It is running the 1.02b5 beta firmware and the fans shut down as they should.
At standby, my unit draws 8 watts. Running with both drives cycling I measured 25 watts. Not sure if the fans were running, that would not add that much so let's ignore that for now.
If I did the math correctly, you would be paying about $2.16/month if the unit was active with the drives cycling 24/7, and $0.69/month if the unit was on standby for the whole month. So your actual cost will be somewhere between those numbers, depending on how hard you are running the system. It might draw a bit more power when spinning the drives up, and there is probably some "worst case" cycling frequency where the drives are consuming more energy spinning up than they are saving by idling in standby...
...let's see...e to the x, dx, dy...oh heck, forget about that. Somewhere between one and two bucks per month...is that close enough for you?