I went back to the same Dunkin' Donuts shop and asked about Munchkin pricing. This is what I found:
6 for $1.50
12 for $2.99
25 for $5.29
50 for $9.49
So, 18 would cost $4.49 and 25 would cost $5.29 (and come in a nice box - the smaller amounts just come in bags). So there are certainly economies of scale there. Still, I think if that were his focus he would have said something like "For 80 cents more than 18 of them, you can get 25 in a box. It's a much better deal, per munchkin." I truly think neither the seller nor the purchaser understood that 6+12=18 applied to this transaction.
