I'm not so sure they are "insanely" expensive to run. Even if it costs several extra dollars per month, you can see what the difference in the electric bill is and pay that part of it yourself - your roommates need not absorb that cost. Also there's a device called a "kill-a-watt" that'll measure the power draw of a device - you could use one of those to exactly determine what you owe for your rodent supply. (You can get one online or I guess Home Depot/Menard's/Lowe's have them these days.)