I put a Kill-A-Watt meter on a mini for about 5 days. Idling along it said 20 watts. When the disk spins up and things get busy, 32 watts. After 68.18 hours, 1.55KWh.
That was with an older 1.66Ghz Intel mini, 1.5GB RAM, upgraded to 120GB Seagate drive, bluetooth on and Airport off (Ethernet connected).
I would guess there's a bit of variation depending on how you use the mini and what's connected. I was impressed at the low number. That and the low noise and great value make the mini a winner to me.
Kill-A-Watt:
http://www.p3international.com/products/special/P4400/P4400-CE.html
BTW - I'm watching an AirPort Extreme now, 14 watts.