Some math, some assumptions, and some generalizations. 80+ is 80% efficient under 50% load, gold is 90%. 10% of 50% of 1000w is 50w. Lets say you use the computer for 8 hours a day at this level, for 365 days. In my area of the US residential power is 5.5 cents per kilowatt hour. So 50 watts x 8 hours x 365 days x 5.5 cents per kilowatt hour is $8.03 a year, or 67 cents per monthly electric bill.
Thanks guys, still figuring out if I need to replace the PSU I currently have anyway. If I do, I'll probably get the HX650 since is almost half the price of the HX1000 and I wouldn't be saving too much. (Because turns out my friend wants to sell his hx1000 for 125$ and I can get a hx650 for 80$)