I have a 9800 GX2. I turn my PC off at night. I usually have my PC (not gaming atm; light load), TV, reciever, a couple lights, and a fan on most of the day. Not paying for heat or cooling atm. My electric bill is usually ~$27. Latest one was $25.
I don't think the idle draw is too bad (the idle heat is ridiculous though). I'm very curious about the impact an idle 5850 would have on my bill...
There are more modern video cards that have low idle power draw and can still play the games with everything turned up.