You mean using a gasoline generator to power the miners? I wonder if using the house electricity isn't cheaper. If it cost more, wouldn't people all begin using generators?
Still, you forget generators are less than 100% efficient. At 30% that 30kW/hr = 9kW/hr for 6 cent. = 1.5 cents per kW/hr + cost of generator which is far from free.
Anyway, your far better off just selling the gas.