Quoting from:
Bonneau Lab
World Community Grid Post - HPF2 Update, Spring 2013
WCG Post - HPF2 Update, Spring 2012

At that rate a quad, on 24/7, without GPU crunching, that earns about 4 days of credit per day would produce about $100 of research calculations per month.
24*30*.035*4=100.8

Figuring the cost of electricity per month per 100 watts of power (0.1 KW) needed to crunch, on average each hour, (beyond the power needed to run the computer for other purposes), and taking the US average $0.12 per KWH (See EIA source. ), the cost is about $8.64 for each 100 watts per month.
0.1*24*30#0.12=8.64

Say this hypothetical quad averaged between 100 and 300 watts per hour just for the crunching, above other uses, then the cost would be between $8 and $26 per month for that $100 worth of time.

CAUTION: Don't try to just plug those numbers into your situation to figure your costs, unless you have a 24/7 system that is comparable, with similar electric rates. Computers that are on just a few hours a day and crunch mostly while the system is used for other things, and have the %CPU time or %processors set low would likely have much lower crunching costs per month. A GPU could change things. All this is hypothetical and some system could be much more efficient, or not.

The point is, that at least for my system it looks like I get a good bang for the buck by donating CPU time compared to donating money to buy CPU time on the cloud. Plus, I know where 100% of my money for this went.