I think it is misleading to use the price of a used replacement camera as a measure of a camera's value, unless that used replacement comes from a trusted source, with a meaningful warranty.
Many prices for used cameras are illogical - they are more likely to represent vagaries in the market than objective factors.
When considering repair costs, I find it useful to think of the cost of rolls of film. If a repair is going to cost me less than X rolls of film, and I'm likely to shoot X rolls of film with the repaired camera in the near future, it seems more clear that the repair cost is worthwhile.