We do Costco, also. We got a Mars variety pack of full sized bars(30 total) for $12. That breaks down to .40 a bar. They have a limit of two boxes. They say per household, but we decided to try again on our trip today, and we still got the discount. So, between two trips, we bought 4 boxes, a total of 120 bars.
That's a great price for us, and they are bars we'd be willing to eat if there are leftovers. (Snickers, Milky Way, Regular and Peanut M&Ms, and Twix.)