My PC (core i7 [email protected]) at idle = ~90 Watts measured from wall
My PC (core i7 [email protected]) at 100% CPU load on BOINC = ~150 Watts measured from wall
So the marginal energy use of running BOINC 24/7 for a month on my CPU is:
150W - 90W = 60W x 24H x 30D = 43200Wh /1000 = 43.2kWh
Based on my energy cost of £12.2/kWh (16cents US) that monthy cost is:
43.2kWh x 0.122 = £5.27 (roughly $7)
As above I have solar so my real profits are much higher, but even though I only mine in daylight (16 hours a day at the moment) the 60 watts of energy to run my CPU is a no brainer even if I did pay for power.
I was going to write some words, but your numbers speak louder than any explanation I could offer! =)