Depends on Power supply .. Drives fitted, etc. I would expect no more than maybe 20p per day .. 50p in 24hrs.
I may be wrong .. just an educated guess.
This table might be useful:
http://www.newcastle..../a/energyrunningcosts
The right hand column shows the number of kilowatt hours used. As Chuck indicates, 12p per kilowatt hour is a 'ball park figure'. So you're looking at around 3p or 4p per hour to run a PC and monitor.