Sometime last year, someone had writeup where they worked out that buying enough gas (I forget if "natural" or "-oline") to mine 1 bitcoin, ignoring fixed costs like the generator or GPU, would cost them ~1.2 BTC. That might change if you live near a oil well/refinery/coal mine, but I'd kinda like to see a statistical analysis of whether bitcoin time-between-blocks varies with time of day based on which areas have excess solar power.