So I wrote a post about my experience with what seemed to me to be innumeracy in a Dunkin' Donuts shop here. Some folks reasonably suggested that the counter person who seemed not to consider the possibility that the customer buy 18 munchkins rather than 25 might just be trying to save her money, since 18 might cost as much or more than 25. So I decided to investigate further.
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.