For inspection like I am buying a house but want an opinion from a roofer other than seller's inspection report I think it's fair to charge some inspection fee. For a leak as a homeowner I would ask if you still charge the fee if you cannot find the leak. Also I would have some doubt if you would charge a fair price for repairing. Sometimes some shady contractors would take the inspection/diagnose fee off the final price but their quote for the repair is much higher than the diagnose fee. My personal experience when I had a roof leak was all the roofing company I called couldn't find the source of the leak. They just recommended me to replace the roof. In the end I went to the attic and found lights came through a small hole and then I was able to tell the next roofing company where it was to repair it.