1996 Volvo 850 Odometer stopped working

1996 Volvo 850 (sister's ride)

Odometer stopped working. Speedometer still works.
Took it to a mechanic who supposedly regularly works on imports. She was told that the instrument cluster would require removal and that the unit would need to be sent out for calibration....cost = $300....

This sounds bizarre to me. In my (non-Volvo) experience, the vehicle speed sensor input usually goes through some type of electronics module that drives both the speedo and odo. Or in the old old days everything was cable driven....

Can someone explain to me how the odo works on this model and if the quoted diagnosis and repair seem reasonable?

Much appreciated!

Derf