I've done both rears on my W124 and replaced whole assemblies for about $140. The fronts are more expensive.
Usually the track assembly components fail, not the motor.
You can remove the door panel and disassemble or replace the offending part(s) yourself.
I believe the parts are now available (for a few dollars instead of hundreds), so you may not have to go out and buy a whole assembly.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car