Unless you are having leaks or some other problem which requires repairs to the AC, I am against converting to R134a.
The reason being is, yes it is cheaper and friendly to the ozone, BUT it does not cool as good as R12. For the Benzs, I am keeping R12 in them as long as I can. I would rather pay more for freon than be sweating inside the car with the "cool" air blowing.
The dealer quoted me around $500.00 to do a total conversion, so a $180.00 (assuming they are using all MB authentic parts and so forth) would be a bargan. Are you sure they dont have some kind of "hidden charge" or did they include the cost of the R124a?