I had an estimate of about $850 for both sides including new struts, spring seats AND bearing plates (independent shop, not a great one, and aftermarket parts, not Volvo).

I chose to replace only the items which had definitely failed, the spring seat and bearing plate on 1 side. Came to about $140 for the parts (original Volvo), $40 for a spring compressor (Sears) and a morning's work on my part. In 6 months I have not had further troubles.

The spring seats fail somewhat often at your car's age. The dealer people told me the bearing plates (more expensive) don't fail usually, but will if a spring seat goes bad and is not attended to.

If you want to restore original handling quality or improve it with better struts you could have the whole job done. I think relatively few shops would be happy to do a partial replace (of just the clearly bad parts) like I did.

On various cars where I wasn't satisfied with the handling and there was nothing wrong with the suspension I went for better tires at the next replacement cycle and got an improvement. I have never replaced struts, shocks, etc., which weren't defective (broken, loose or leaking).

There is a website where someone has posted strut/spring seat replacement and used the 850 as an example car. You can find it w/Google.