What should my utilities cost on average for one month?

I live in a very very small 3 bedroom house (my room was used as a closet if that tells you how small it is) and it is $650 per month to rent but it is split between myself and 2 roomates. My roomate claims our utilities all together were over $350 for one month. We do not turn on the heat (even in winter) and we have basic cable, no internet, no phone, so all we used is basically electric and water. On top of her turning the heat off on me, she bought 5 space heaters to "save energy."

First possibility - you're getting lied to. Ask to see the bills.

Second possibility - you have something really messed up that is using so much cost. Probably not water (flat rate usually except for gross over use), probably not gas heat (as you use so little), you can find out cable costs online and you have no phone. Doubtful trash costs so it is probably electricity.

Gas heat is more efficient than electric heat. Space heaters are not efficient and probably cost more than using the whole house heat or a wall heater. The efficiency of space heaters varies widely and it is definitely worth the investment to by the most efficient space heater possible.

Then seal cracks under and around doors to prevent heat loss. Blankets hung over single pane windows can save very much heat.

Other sources of electric waste : old refrigerators.