It depends on the temp/size of the room you are trying to heat. We heat 2-3 rooms with oil filler space heaters and it cost about $500 for electric. (That includes standard household electric use too.)

When we hooked up another room in our basement for breeding it was a cold room and it raised our electric bill $120 a month for a heater that ran at all times. We used it for a month then switched to a upstairs room and it is a lot cheaper now.

It really depends on what your electric company charges and the room itself.