The kill-a-watt would be neat to do, but a lot of caveats:
1. My 110 volt/10 amp 21 inch mower is probably more powerful than the typical cordless. My grass was lo-ong, and this thing chewed it right up. My old 4hp gasser would have struggled with it.
2. Being more powerful means I don't have to mow as often
3. Batteries don't need replacing on a corded, and less initial cost.
4. The used corded models are really de-valued right now. I don't think you can get a used cordless w/decent battery for anywhere near the price. This one in MN is $10!!
mongomery wards electric mower mulcher bagger. - eBay (item 250301427137 end time Oct-05-08 15:57:36 PDT)
If it is a question of efficiency, watt for watt, I still have to put my money on a corded model. chargers and batteries are not %100 efficient. An extension cord isn't either, but it is a much more direct way of powering the motor.
For Convenience, the cordless gets a point, though they are a little heavier to wield, and they can only do half a lawn at a time around here, you wont have to worry about accidentally dragging a cord over your begonias
If it is cost, initial and ongoing, the corded starts out ahead and keeps getting farther ahead.