A cashier can process tens of thousands of items per hour, at a wage of under $15 per hour. This means that the cost of a cashier can be around 0.1¢ per item sold for a busy store
Generously assuming a cashier can check one item a second for an hour, he can check only 3600 items. "Tens of thousands" seems off by at least an order of magnitude.
I was thinking of some costco cashiers I've seen, and they do far more than one item per second. That said, when you consider the downtime of processing the transaction, your estimate is probably not too far off. It makes the low end cost of transponders comparable, though just.