Power usage, of course, varies between different computers, one of the main things that uses electrictiy is the monitor with old CRT type monitors using a lot more than modern LCD ones.
taking a fairly average system (with a LCD display) the typical power usage is about 200watts while on, about 20watts with everything on standby (sleep) and about 10watts with everything turned off.
to give a "real world" comparrason to keep a computer in each of the 3 states above for 10 hours (based on a electricity unit cost of 12p) would be......
on - 200W = 24p per 10hours
sleep - 20W = 2.4p per 10 hours
off - 10W = 1.2p per 10 hours.
of course unpluging or switching off at the wall would reduce the above to 0p