I replaced mine myself. The part cost was $230 or $240 from a local automotive supply (not a dealer). I would guess that the dealer price for the part is a bit more, so the $420 that pepperish paid seems pretty reasonable. The job isn't terribly difficult and probably takes a Volvo mechanic an hour or less.
'02 Volvo S60 AWD (traded in)
'10 Dodge Avenger R/T