Electricity costs will vary quite a bit. Here it's 7.6c/kWh. I have seen power bills as low as 4c/kWh and as high as 15 or 20.
One thing to note... your computers may have power supplies rated for 300W, but they will rarely use that much. In fact, the max wattage pretty much is only ever reached at startup time. I know my gateway/router machine is quite low power (I will have to get a proper wattage meter but I believe it's about 30W or so). It uses a VIA C3 CPU. That along with my fileserver, workstation, & monitor form the majority of my electricity usage (which is about 650Wh average). I have other computers but I turn them off if I don't use them for more than 24 hours. They don't really use much.
In August/September, I had the AC on a lot. My average was 1.25kWh. Pulling it out and just coping with the heat cut the bill by half. Just that one thing!
Anyway, my September energy bill was $61. I expect October to be more like $30 or $40. So SiN's numbers are reasonable for a small apartment like mine. You can expect to see even smaller bills if you're particularly stingy about your power usage.
Fortunately, with this core set of computers, my apartment remains quite warm. According to the thermometer, it varies a bit between 75 and 85 F... that's with no heater running.
edit: I forgot I wanted to mention that I use low-power light bulbs. They are efficient versions of incandescent bulbs and I can't tell the difference between them and the normal ones. I replaced 100W bulbs with efficient 25W bulbs, and 40-75W bulbs with efficient 18W bulbs. I'm sure that contributes quite a bit to the power savings. In addition, they last much longer than normal bulbs. I have a friend who uses similar bulbs and he says he's still using the same ones after 7 years!