I've heard arguments both ways. I think the big difference is you are basically paying for the cost of about 200-300 flights up front with an electric, while with nitro, you can buy your fuel as you go. A Lipo battery pack lasts around 200-300 cycles... divide the cost of the batt pack by about 250, and you get your cost per flight. Compare that to the cost per flight for a nitro plane of similar size and performance, and I think you'll find the electric may be cheaper (although gasoline is probably the cheapest). Problem is... if you crash and trash your LiPo pack after say, 20 flights, then you only amortized the battery cost over 20 flights, instead of 250!

You can also factor in the cost of the electricity to charge your packs if you want (assuming you charge them off of a DC supply plugged into an outlet at home), but that should be minimal, if not negligible. For example, with a 6lb plane, you'd probably run around about a 600W power plant, and a 10 minute flight would burn 0.1kWh of electricity. It will take a little more than 0.1kWh to recharge the battery due to inefficiencies, but you get the idea...