It's not that expensive, at all. Costs about $1.00 to run the computer for 24 hours, so if you're using it for 12-16 hours out of the day like Joey probably is, you're only saving $.30-50 cents, and the monitor is what's usually taking up most of the juice anyways.
It's more about convenience for me - even if startup/shutdown time is minimal, it's just way easier for my lazy ass to turn on/off my monitor.
The 'always on' mentality was fostered 5-6 years ago, when it was actually better for components to be running all the time instead of being turned on/off, as the heating/cooling differences stressed things like solder joints and sockets.