With respect to the issue of job estimates, there are some critical points that need to be made “up front” in order to minimize the possibility of any misunderstandings.
First of all, it is virtually impossible to give any kind of accurate estimate without inspecting the unit first. I absolutely CANNOT make an estimate based upon verbal descriptions of a problem. This is especially true for older amps where the problem issues often involve cleaning/maintenance work in addition to correcting any malfunctions resulting from faulty/failing components or connections of one form or another (tubes, capacitors, semiconductors, hardware, etc.).
       
There are also instances where the unit may have multiple problems, which might  not be apparent until the more obvious issue(s) is first corrected.

Another potentially problematic situation is when the unit’s issue is found to be intermittent.