Actually, you should be able to figure this out. It sounds like your machine has a power rating of 600W. Energy companies usually charge by the kilowatt-hour. Multiply 600W by how many hours you have your computer on each month. Divide that number by 1000, and that will be your energy usage in kilowatt-hours. Then just call up your power company and see how much they charge per kilowatt hour, and you should be set.
For example, if you run your 600W computer (without sleeping, or in a reduced power mode) for 24 hours a day, 30 days in a month, then you will use (600 x 24 x 30)/1000 = 432 kilowatt-hours. I believe my electric company charges $0.0396 per kilowatt-hour (April - September). So the charge would be $0.0396 x 432 = $17.11. (that is for the computer only, not including the monitor)
Now, the eMac is rated at 170W, which would come out to $4.85 (and that does include the monitor). So, it is possible that your new PowerMac is running you that much more a month. It really depends on how much you are using it, and how much energy it uses when it is asleep (I don't know that).
Tamara