not a lot at all. my husband runs a business doing gauges, calibration and instrumentation, and the PAT(esting) is a tiny element that makes hardly any money for the time it takes and travel involved sometimes.
You could do it freelance (buy the equipment yourself) or work for a company that do it, so its up to you if its salaried or not.
Freelance, perhaps have a minimum call out charge to make it more worth your while, but if you work on an average of �1 per item to be competitive, you're probably in the right area. You will also have to provide a certificate to say it has been PAT.
Salaried, its not highly skilled, so less than �20,000 if thats the sole task involved!