[premchai21]

I would think that a significant part of money-handling costs (in the general case) is mitigating risk of loss of money through fraud, theft, and similar means. That risk would have to be very small to be ignorable at scale. If you process ten transactions of a thousand dollars each and have to refund one of them out-of-pocket because it was invalid and the thief moved the money out of reach, that's a lot different from processing ten transactions of one dollar each with the same per-transaction risk. It makes sense that this would lead to amortized risk-level × transaction-amount costs.