Now, that's something you can afford to leave on all day crunching work units.

http://www.theregister.com/2004/08/30/orion_delivers_personal_cluster/

it costs 10,000 dollars though.. so that offsets the power saving cost.

I think 12 machines would eat 2400 watts of power, (estimating 100% cpu usage, 200 watts), so that's 24 cents per hour in power cost (10c per kWh estimate) or $2100 dollars in power a year.
(assuming you're leaving the cluster on 24x365)