I'm no expert at it, but with the efficiency, if your computer actually USES 300W, then it would be whatever the loss in efficiency is plus the 300W.
70% = 390W (would computer would be pulling)
75% = 375W
80% = 360W
85% = 345W
At $0.10 per KWh, 12hrs per day, 30 days per month...
70% = $14.04
75% = $13.50
80% = $12.96
85% = $12.42
Heck, I could be totally off base...
Now, if it's pulling the 300W, need to know what efficiency you're currently at to make any guesses :D Looks like every 5% you save about $0.50 per month.
Get a Kill-A-Watt and find out exactly what it pulls, and use this calculator to see what you're paying...and I take no responsibility for it's accuracy http://www.maximintegrated.com/design/t ... y-cost.cfm