No one ever went into academia for the money. How much do you pay for premises and other overheads such as staff you employ or energy bills etc? Like the amp tech likely does, I mean. $50 to bias an amp is a fair chunk, and to judge its fairness I would take into account the reputation of the tech, his location - garden shed/ within a retailer, whether it was done on-site or shipped, turnaround time, any ancillary checks on amp serviceability thrown in. I would also factor in what it was worth to me in time saved and peace of mind.