My power bill (with 12 computers, AC running full blast, 12 monitors (6 of which suck 700+ watts in total) and everything else went up about $400 a month. Where I live, power costs around ten cents per KWH. I have found when I don't run but my main machines and use powersave modes on my monitors, it costs about $150 more per month. Granted, these were in the middle of summer so AC was running full blast and AC takes A LOT of power.

There are ways of doing this for far less power and keep in mind this is an overkill setup. Most people who add 5 computers and a bit of cooling will bump up $100 - $150 a month most likely, depending on power save modes, use, if you turn your computers off or not, etc.