Are you folding with your computer? You don't need to leave the PS3 or computer on, as the folding works in the background when you power up. Below is somewhat old cost data for a computer if you do leave it on. I assume it's about the same if you leave the PS3 on.
http://folding.stanford.edu/English/FAQ#ntoc55
How much power/money is used by keeping a FAH running 24/7 on a computer?
Roughly, a CPU uses about as much power as a watt light bulb. Here's a report on computer power management from Lawrence Berkeley government labs, and there are other references on the web you can find. Although power supplies on most computers are rated at 400 watts, average usage is lower. On average, a Pentium-type computer uses about 100 watts (if the monitor is off). So, the daily difference between off and running FAH is about 24x100 = 2.4 kWh. At $0.15 per kWh ( from PG&E here in California), this works out to about $0.36 per day. In general, lighting and climate control use a much larger share of household power than computers do. So the best bet for cutting costs and conserving energy would be to turn off lights, turn off your computer monitors (which use more power than a CPU), and turn down the heat.
What is the power consumption of the PS3 running the FAH client? Is it safe to run the PS3 24X7?
We expect the PS3 to use about 200W while running Folding@Home. We have several PS3's running in our lab, running Folding@Home 24/7 and have had good results so far.
Granted, this is likely based on the original, 60 gig model (the one I own), but that is a massive power draw and far more than my dual core PC draws when running F@H in the background.