I dont know how much they make, but my experiences with dealer service writers and parts people are night and day.
Service writers are obviously on commission and dont seem to really know much about the cars and trucks, just selling. The ones at the local Sears do seem better than average for that type of place.
All of the dealer parts people around here are super. They know the product and will take the time to research something that they do not know. This applies to high end, BMW and low end, Ford or Dodge, equally.