This should give you an idea about the cost. Converting a .40 size plane to e-power isn't going to be cheap. You are looking for a 600 to 800 watt power system.

http://www.hobby-lobby.com/g2e-sigrascal40.htm

You will need about 3 of those $160 lipos to to rotate off the charger if you want to do a reasonable amount of flying. ThunderPower OKs a C-3 charge rate for some of its lipos (20 minute recharge) only if their charger is used along with a balancer. An amp/watt meter is good to own, no, make that vital to own, unless you want to give electric power a bad name. Without an amp meter, your only clue that you put a too large prop on the motor is smoke or short motor/esc life.