My rule of thumb is, does the fix cost more than buying an equivalent? If so, I have struggles paying more for the fix.
For example: I have a Pfaff Hobby that they said service and fixing the timing would cost 400$. Ummmm Hello that was more than I paid for it twice over! So it became a door stop for a bit, then it found a home with a woman who wanted a machine who's hubby was a tinkerer. She is thrilled and he is able to maintain it for her.