So the trick to maximizing profit is to mine efficiently. Mining is very simple math and you can run your cards at much lower voltages than is "gaming stable." The most profitable way to mine is to keep your core clock stock (925MHz in my case) and lower the voltage as far as it will go stably (0.931V in my case). Mining performance almost entirely depends on the core clock speed, so you then can reduce the vRAM to ~300MHz (unofficial overclocking in MSI Afterburner helps here). Using these adjustments, my 7970, which loads my system at ~275-300W from the wall during gaming only consumes ~180-185W mining with these adjustments. Now I actually run my clocks much higher in the winter because I use the heat to heat my condo.
Anywho, recalibrating your usage, you'll probably pull ~500W under mining load. The average cost of electricity in the U.S. is ~$0.145 per KWh, so assuming you mine 24/7 in this example, that's $1.74 a day or $12.18 a week. Mining used to be more profitable, but prices are rising again so it may become more profitable once again. Still, that's $15 profit a week for just letting it sit there. Check out the thread on AT:
http://forums.anandtech.com/showthread.php?t=2167303