The marginal rate within a certain tolerance is reasonably flat, of course, but in a slightly broader perspective, it's not. It's obviously not a clear-cut line

The marginal rate within a certain tolerance is reasonably flat, of course, but in a slightly broader perspective, it's not. It's obviously not a clear-cut linear rate (like with CCs), but handling more cash is certainly more expensive. Insurance, mistakes and "wastage" all increase as the sums of money handled goes up. Also, more money takes more (expensive, background-checked) people to count, and it means bigger safes and more frequent pickups.