Here's a website I found which details expected cab fares around STT. I thought taxis were a bit pricey (considering the short distances) but part of that is because they charge per person.
http://www.vinow.com/stthomas/gettin..._stt/taxi_stt/
Guess I'd try to estimate where you plan to go each day and how often, then decide if $50 for a car would be less. One thing I found - the big car places there (i.e. Hertz, Avis, etc.) are mostly local franchises and none of them accepted any of the coupons, discount codes, etc. that I had.
I did rent a car while in STT and enjoyed the freedom it gave us. Some other things to consider... They drive on the left there, but unlike in the UK, the cars there are all left hand drive (all shipped in from the US). A little weird but you get used to it. Unless you really really need a big car, smaller is better there. Not only is gas very expensive, but the roads are very narrow. They are also very hilly and quite often, very choked with traffic. It can take a long time to go a very short distance in STT. Also parking in Charlotte Amalie is pretty tight too.
Think you are correct about the hotel shuttles. Don't remember seeing any of them at the airport.
