I doubt if you would find a taxi willing to do that trip for less than 250 - 300 Euros. And you might find that many of the taxi drivers do not speak much English. The price you were quoted is pretty typical for that type of tour which involves an entire day with car/driver. You need to understand that the lousy value of the US$ and gasoline that is now about $9 a gallon is not helpful when trying to make a deal. Probably the most economical way to do your itinerary is to get a group of 8 and hire a van/driver. It would still be expensive, but the price per person might drop to under $100. Keep in mind that if you are traveling during the very busy summer season the traffic on the Amalfi Coast can make the drive from Naples easily take 1 1/2 hours each way.