This question really interested me because I'm quite a fan of saving power, and I happen to have an inline power meter.
So on my HP Laptop while connected to my Monitor it's drawing a measly 24W of power so over a 24 hour period I'm using a measly 0.576 kWH and over a year that's just 210kWH.
Now in NZ we pay 25 Cents per kWH so over the course of a year the laptop costs me $53 to run, not bad.
Interestingly it uses slightly more power if I have the display on, around 30W of power and if I do a Defrag or force the HDD to do some work it jumps to 40W.
Either way if you had a machine that was getting used quite heavily you'd still be only looking at around $100 per year.