When I worked dive shops as a teenager, regulator overhauls were about the least desirable, least profitable activity to be had; were frequently labor intensive, since most everyone back then, barely rinsed the damn things. The lowest guy on the totem was often tasked with the work, removing surface corrosion with hempocid and a spent toothbrush.
There was an initial evaluation of a regulator (a simple bench test) upon arrival; a disassembly of the stages; ultrasonic cleaning; rinsing / drying; reassembly with new parts; a second bench test and final tuning. If further issues occurred (an unstable IP, a bad seat, etc), there was a second round of disassembly / reassembly; and if the tech was worth his salt, that equipment didn't leave the bench until it met or exceeded the manufacturer's specs.
On a troublesome regulator (what Vance Harlow once referred to as "possessed" in his classic regulator book -- and I saw a few of those over the years -- that could prove to be hours. There was also the "idle" time spent with the regulators, sitting under pressure, often overnight, to verify a stable IP, before any release.
Classes and new gear -- later, travel -- were where the money was to be had -- seldom the maintenance of that very gear that they sold . . .