is it necessary to replace the seat belt pretensioner after the airbags deploy (in a 1995 volvo 960)?
how much does the seat belt pretensioner usually cost?
In general, yes. The seat belt pretensioner fires along with the air bag. You would not be able to tell from looking to see if it has not. So I would change it when replacing the air bag.

Highway deaths have plummeted to their lowest levels in more than 60 years, helped by more people wearing seat belts, better safety equipment in cars and efforts to curb drunken driving.
Seatbelts are designed to protect, but as one industry spokesman put it, "They are designed to work just once." The question of whether to repair safety equipment after a crash raises another issue: What about the air bags? How do you know that once they have been deployed they are being replaced correctly?