I'm not quite following your objection.. I'd expect a test that checks the multiplier is correct would detect orders of magnitude discrepancy. So if you're billing $x/byte you'd write a test for the billing thing that checks that, given y bytes, the bill is x*y.
[edit] This may need to be an integration test to be effective, there is a certain peril to mocking that could bite you here. But that's fine, we have the technology.
[edit] This may need to be an integration test to be effective, there is a certain peril to mocking that could bite you here. But that's fine, we have the technology.