LiPo batts should last about 200-300 flights, so divide the batt cost by that to get your cost per flight to compare with fuel costs. Problem is, if you damage your battery pack in a crash, or by over-charging or over-discharging before the 200-300 flight lifetime, your cost per flight goes up accordingly.

Basically, buying LiPo packs for your planes is like buying fuel for 200-300 flights up front, and that's quite a bit of nitro!