Hi Mike...

try this site....

http://fcpgroton.com/VolvoAC/volvo850ac.htm

It's listed at $47.50.

Unfortunately, it will require opening the system to air. I have opened systems up briefly to replace switches on other cars during the course of my life, and have immediately then pulled a vacuum on the system, and held it for half an hour before recharging, in an attempt to get away without changing the accumulator/drier. Got away with it about 95% of the time, but I hope a more experienced A/C type responds to this question. I suspect that most will offer the opinion that any time you open the drier to air, the dessicant becomes saturated with moisture. But a very brief exposure, followed by immediate evacuation, may be acceptable to many, and works for most.
If you're going to try the "quick fix", be sure that the system is warm before you open it (to prevent "instant" condensation), work fast, and have everything hooked up so that as soon as you start putting the new part into place, you can start the vacuum pump. Do NOT start the pump until the new part is seated, or you will draw ambient air into the system.
By the way, intentionally venting Freon to atmosphere is not legal in most jurisdictions. And the system should be bled down to near zero pressure before you remove the old pressostat, or the sudden release of gas can be dangerous.

Good luck

Marty
'96 855T