I had assumed the cost would have been related to the volume of the coin storage in the machine relative to the associated stock.

I probably should have said "hypothesize" rather than "conclude." My memory from when I operated an (antique) vending machine was that it had plenty of wasted space. I certainly wouldn't want to introduce a machine accepting pennies into the existing world in which they are otherwise useless. But if storage is an issue for the machine, it's also an issue for people. I don't normally carry any change.

Another source of information is automated coin counters. I had one bank that offered the service for free to customers and another that charged a percentage. I've seen a free-standing one (a vending machine that takes pennies!) that charges 15%. So maybe Tailsteak should discount the number by 15%, but no more.