The biggest issue i see on the hard ocp review, whats thier voltage? 1.4, 1.5, 1.6?
http://www.tomshardware.com/reviews/bulldozer-efficiency-overclock-undervolt,3083-18.html
Maximum pull during the test was 350 watts for 5-8 seconds, while most of the time is spent below 250 watts.
Toms used 1.5v for their 4.6 ghz oc ... imo thats friggin insane. looking at the chart above, their 4.0 ghz was at 1.38v, never peaked over 250W other than a split second at the end.
Overclocking BD doesn't require that much voltage unless you don't know what your doing, or want to prove that you can drain some power.
my overclock is 4.7 ghz at 1.366V on this asus board and i know im not the only one.
My friend has his at 4.5 ghz at 1.33v on his gigabyte board on the stock cooler.
As for your question, you might see $2-3 max per month over running an i7 2600k during gaming sessions if you pump the voltage up to 1.45v as most of the tests out there did. running at 1.36V, maybe not so much of a difference, but ~$1-2
I don't have a wall amp meter but i know for a fact that im not pulling nearly as much as the tests show.