how often would a 1984 volvo 760 turbocharger have to be changed?

Question

how much would it cost to have repaired?

Answer 1

i remember replacing several of these[that had seized up] with ones that had water cooling kits, but for the most part the turbo's lasted the life of the car [once in a while a shaft seal would start leaking and clouds of blue smoke would occur] this was hard on cat converters.
as far as prices go re-man's seem to go for 800 or so and new and larger units are in the 12-1400 dollar [us] range.
the key to life for any turbo car is frequent oil changes and waiting a Minute or so before shutting down.

Answer 2

You rebuild it every 100K miles (assuming that you change oil every 3000 miles) You can extend the life of the turbo by changing oil more frequently (every 2000 miles or 1000 miles). You can rebuild it yourself or have it rebuilt by a professionals for $300 - $400. Or replace the turbo with another rebuilt unit (core charge applies)

You should be able to find rebuild steps by searching internet. These Garrett T series turbos are very common and Volvo is not the only car uses Garrett turbo. Rebuild kit runs around $80 and can be done within an hour.