We're currently doing an audit, non-security audit though, but auditor salaries will probably not deviate that much. We hired a top-five audit firm and were billed roughly $400 per hour in total for two auditors (senior, junior) plus some work by the partner, mostly at the beginning and end.

The scope of their work of course is what drives the total cost, but a single full-time week would usually range $15-20k.