I've bought a socket current meter to measure how much it costs to keep a pc running 24x7. I just plugged in my Shuttle pc and discovered that it consumes just under 90 watts when idling. Running at about 50% CPU this rises by about 5 watts then up to about 100W at 100% CPU. So left on for a year, sitting there idle most of the time, we're looking at about 800 KWHrs or £48 at 6p a unit (the price on the most recent bill I could find).
The Shuttle also has a standby mode where power consumption drops to about 50W, which is more than I expected.
I also measured a Dell Precision 340, which has a faster Intel processor than the Athlon XP2200+ in the Shuttle, and this registers about 80W when idling. However this jumps up to 130W when running at 100% CPU.
Finally the one component I currently keep running all the time, the ADSL modem/router, consumes about 12W, a small cost to pay given that this is sometimes flaky when starting up from cold.