Keep in mind service recommendations for maintenance are just that "recommendations" . As the old saying goes your mileage may vary. Depending on your driving conditions you may actually need to replace the plugs/coil before the 100k service recommendation. 85k miles is pretty good for plugs/coil.
If you think your dealer is charging to much first get in writing what they will actually be doing as part of the "tune up", call a few other shops and compare. At the end of the day if it's too much for you you do have the option of doing it yourself....