This topic seems to pop up fairly regularly, and usually with the same result.
It has gotten cheaper to fly electric, but it is still much more expensive to power a .40 size trainer with electric.
As Dennis said "Apples to Apples". A comparable electric trainer would need to be able to fly for 15-20 minutes, and I believe this is where the major $ difference is. To get that kind of time you'd need a whole stack of Li-poly's, and that kills the price.
The gap between electric and glow/gas is narrowing very quickly, but $ still seems to be the biggest hurdle.

Jason