I've never had the inclination to figure it out before either ...but my employer does in fact pay me a substantial amount in exchange for the time it takes to do more than the simple math in this thread in order to advise when our heating plant should switch from oil to NG and at what rate to phase out ALL electrical HVAC appliances at a facility that spends 27 MILLION dollars a year on electricity (and we heat with gas). For the home user with a demand for a minute amount of energy by comparison I will agree that gas (NG or propane) is pretty much on par with electricity, but I stop short of saying electricity is substantially cheaper because it is not, it is my job to know that and I see the figures to back it up quite frequently. The main reason I wouldn't suggest switching a home from electricity to gas is the fact that the price fluctuates so much (we're set up to switch back and forth, homeowners are not) and is damn near impossible to predict what it'll cost 5 or 10 years down the road. Also, the initial cost to switch everything over far outweighs the savings for the average joe. Geothermal, wind, solar, etc is the same story but at least with those you have the satisfaction of not handing your money over to the big corporations.
Efficiency is a different story and John is 100% correct in saying the electric brewery is more efficient and the cheaper option (compared to the propane burners), but it's only cheaper partially because the availability of components for you and I to build a high efficiency gas unit isn't yet available. Careful about what you assume to be high efficiency when talking about electricity because as soon as you convert electricity to heat that all goes out the window (Heat is waste ...a by-product of sorts, and therefore the opposite of efficient from square one).
My point here was and still is - it is not substantially cheaper to use your kitchen stove rather than a turkey fryer to boil 5 gallons of wort.
My intent isn't to start a big debate, and I'm far from a beer making expert, but the certificate on my wall and my employer seem to think I know something about energy, efficiency, and mechanical engineering. Personally I think I've got them all fooled as I'm just another dumbass stumbling my way along ...but I digress