There's no single honest number here, because cost depends on what's being tested, not a flat rate any legitimate tester can quote before they understand your scope. But there is a real range, and quotes that dodge the question entirely are usually avoiding it for the wrong reasons.
This page breaks down what drives the price, gives realistic ranges by engagement type, and explains what should be included in a quote versus billed separately, so you can tell a fair price from an inflated one before you get on a call.