"up to..."
Some audits are completed on-site in much less time than that and they require only minutes to report. On those occasions, the pay is okay. You never know in advance if you will be able to complete the job in a shorter amount of time, so the pay should be adjusted up slightly to account for variations. The pay should not be so high or by the minute, to prevent clever auditors from dawdling in order to get more money.
Nature does not hurry, yet everything is accomplished. - Lao-Tzu