I am planning to use Atom SIngle + ION as home server. How much extra $ should I expect per month?
the FitPC2 is running at 6-8 watts or 4.32 - 5.76 KWh per month), it's doesn't get much better. the Intel GMA500 is said to play full HD (but that's not really a requirement for a server, or is it?).
the Acer Aspire Revo, as a comparison, with it's ION GPU is consuming 22-27 watts (or 15.84 - 19.44 KWh per month).
so the difference would be about 11 - 14 KWh per month.
now the 'extra $' depends entirely on the rate your supplier is charging. :)
mind you, those figures are based on 24/7 operation and do not include costs for a monitor or external hard drives you may connect.
As a rule, given electricity prices of $0.14 USD (avg is $0.12 now), take the wattage of an appliance and divide by ten to get a dollar value for cost of electricity each month. An iBook using 30W will cost you about $3/month.
Molly's suggestions of the FitPC2 and Acer Aspire Revo would cost in the neighborhood of < $1 and $3, respectively.
Two biggest sources of energy waster with a home server are: displays (esp CRTs!) and old hardware. Using a slow 486 with a 13" CRT might seem like a good way to be cheap, but it's going to be costing you $10-20 in electricity to run a month.
Regarding how much electricity costs per month (if you live in the US) see: http://michaelbluejay.com/electricity/cost.html
The info is very easy to find- just Google for something like "average cost electricity $locale", where $locale is where you live.