I bought a Kilo watt meter a few years ago and spent some time tracking energy consumers. I have allot of computers running in my home (I work in IT) and I also have fish tanks, plus I live in an apt in CA (Worst power prices ever). I tracked these devices over at least a week of average use, most of them a month, then did calculations to find out what they cost per month.
My 75G fish tank with 4x54 w T5 costs $15 per month.
A server (Like a dell poweredge) costs about $35 per month
A standard desktop computer running 24/7 (No sleep) costs about $20 per month (Not counting the monitor)
My Laser printer which is well used (~500 pages per month, I'm in school as well) costs $1.87 per month.
My laptop if left to run 24/7 with no sleep mode costs $0.57 per month.
My 25G tank with 2x55 w PC cost $7.50 per month before I replaced it with the 75.
Hope this helps!