Hi Mike,
I'm sure things are different everywhere, but the outside inspectors that come and subcontract for us here usually send me a bill for their services. I've seen prices like 30/hr for visuals and 45/hr for UT work plus motel & mileage. This seemed pretty common among the inspectors that come through our shop. We aren't a bridge shop, but we do similar work. I don't feel these wages are too out of line for the type of work we do, but if it were my only income and I had to buy health insurance and all the expense of running a business, well, the price would probably be forced upward to take those things into account. But for side work, I think it is reasonable pay and will likely get you repeat work. Email me ( jwright650@aol.com )if you are interested in this type of work as I have promised a guy (maybe the same guy who contacted you) that needs inspectors for part time projects(4 weeks here, 6 weeks there, type of thing), I would send him CWIs wanting that type of work, for him to review.
John Wright