Holy moly - 900 euros for that trip? Ouch. There are areas and cities where a tour can be very beneficial but the Amalfi Coast doesn't need a tour as such - yes, a private car from Naples to the area can be a nice way to explore the area when there for just a day but keep in mind, most of the villages are off limits to cars so once the car gets you there - you end up walking anyway so why pay such an exorbitant amount of 900 euros (look at www.xe.com to see the currency exchange for that amount).
I've take the train from Naples to Rome for 19 euros (I think now the lowest may be 29 but I haven't check of late) when bought in advance; 70 speedy comfortable minutes - then the private car picks you up right at the train track in Naples and off you go.
Save some of that money for the good stuff - food, wine, goodies of other kinds.