My fancy new iMac is really power efficient. It averages about 70 watts when I'm using it; that's about like a lightbulb. Impressive, considering it's a 27" LCD monitor. It's less than half my old Windows desktop and about the same as my stupid idle A/V rack. Assuming I put the iMac to sleep half the day, that works out to 25kWH or $9 a month.
The iMac is basically a laptop bolted to a giant LCD monitor, with efficient mobile CPU and GPU keeping heat and power down. The biggest constant power draw is the LCD backlight; the automatic dimming helps quite a bit. Here's some numbers (top end 27" iMac: 3.4GHz i7 quad CPU and AMD 6970M GPU).
As efficient as the iMac is, it's still worth the trouble to let it automatically sleep when you're not using it. I've yet to have a Windows desktop where sleep mode actually worked; totally solid on the Mac, of course.