Bill,
I am not sure what was in the Cat estimate. It seems very high. Perhaps oil analysis, chassis lube, fuel filters, and perhaps synthetic oil among other things. Basically the cat oil filter is about $30, oil as Marty said is about $130 for non synthetic diesel oil like a Rotella or Delo. A Cat fuel filter, if that is changed is about another $25. If the Racor is changed probably another $50. Labor for a good job would be about 2 hours with a chassis lube. Be sure that the shop that you use is familiar with what fittings should and should not be greased on your particular brakes. Some air brakes have fittings that require special high temp clay based grease.
Add a fair mark up on the oil and filters that will be changed and two hours of the shop labor rate and that is about what you should pay.
Later Ed