Nearly zero. The only thing that delivers power while in sleep mode is the +5vSB line and that usually only gives out like a maximum of 3A. That adds to like 15w maximum.
The power bill comes in at... 10 cents per KWH high estimate, at least 66 hours per KWH, no more than about a cent every 6 or 7 hours I would guess. Probably even less than that.