Based upon the findings here . . .
http://living.morethan.com/2007/12/13/it-all-a dds-up/
. . . a typical PC in an office environment (9 to 5.30, 5 days per week) uses 2.29kWh of electricity per week. Scaling up to 24/7 usage gives a total figure of 7.87kWh per week.
So that means that just over 1kWh is used per day. Tariffs vary but (after paying a higher rate for the first dew units), British Gas charge me 9.65p (+ 5% VAT) per kWh. Using that office PC (24/7) would cost me
about 11.4p per day.
Of course, if a PC is turned on, the monitor will normally be turned on as well. Assuming that it never goes into standby mode (and that it uses the 20W, quoted for a 19" flatscreen monitor, referred to in that article) it would cost
about 4.9p per day.
Chris