Are you sure he said "pinion gear", and not "pinion factor"? That's the name of the parameter in the PCM that controls the speedo reading.
As far as the $80 goes, well... you don't expect them to do it for free, right? Maybe their policy is 1 hour shop time minimum. I agree it might be a little excessive for a 5 minute job, but they're the ones with the $4,000 computer.
Have you checked other dealerships?
-Tom