The street price for R134A is around $8 to $10/# plus labor to install. You are getting a bargin for $85 if he is doing it right.
Generally, adding refrigerant to a sealed refrigerator system is a waste of time and money. To properly do it, he has to add solder in Schrader fittings, and evacuate the system. I would also want a liquid line filter drier added at the same time.
I would expect a quality job to be about $300 plus/minus.
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