How much would it cost in electric to keep a tortoise?

I really want a tortoise and ive been searching on the internet and looking up to find out how to care for them and i know that if you want to keep one inside then they have to have a tortoise table thats heated and with a lamp but how much in electricity would that cost? i know tortoises are expensive and i would be able to buy the tortoise and everything it needs but i just need to know how much the electric would cost each month for using the tortoise table. any advice would be much appreciated (: thanks