The space heater is what kills. All my critters are in my bedroom, so the heater would have been running anyway. My mom complained about it (I'm yet another college grad begrudgingly moving back in to the family home to look for work), so I sat down and worked out how much I owed her and how much was just a normal spike in cost for heating a 100 year old house.

What I did was multiply the wattage of each heating element (heat pads, lamps, etc.) by the number of hours they run. Multiply by the # of days each month. Electric companies charge by the kilowatts per hour, so multiply by your kilowatt/hr rate. Then divide by 1000 to convert from watts to kilowatts. All of my critters (frillie, 3 snakes, leo, and a small dubia colony) only came out to about $15/month at a rate of $0.12/kilowatt hr.

(Wattage * Hours * Days * Rate) /1000 = monthly cost

I might be COMPLETELY wrong on that formula, but it seemed to satisfy my mom at least. XD