Manufacturer inspection cost

i've read many people send their eqiupment to be "checked" by the manufacturer. How much might this cost? Why would this be necessary if the equipment runs fine?
Particularly tube amps degrade very gradually, so that it is difficult for the user, familiar with the sound, to determine when this degradation becomes serious. Objective measurements, which the manufacturer is best qualified to do, are therefore appropriate.

Transistorized amps, on the other hand, usually go "thump!" and blow a fuse. Hard to miss.
I always wonder if it didn't fail and go back for repairs when this is said. If the unit workes fine, why would you send it back otherwise??