You can rent a car for a week for as little as $31, if you book it online. I didn't know this until I read it on the Cruz Bay Forum at TripAdvisor.com. The going rental rate for a car on St. John is $60/day.
The discount is from Priceline.com. Thrifty is offering deals ranging from 
$254/week for an economy car, including taxes and fees, up to $270/week for a full size vehicle. Of course, you also have to add
the cost of a car barge round trip from Red Hook to Cruz Bay. But then, you're not paying for a taxi and luggage to and from the airport, and that just about makes up the barge fee. It may even be cheaper if you've got four people. (Click on the photo at right for a bigger picture of the rates.)
Now, I'm not encouraging this. And neither was one of the forum's members,"isle263," who wrote "Cheaper????? What if something happens to the car? I don’t think they will drop what they are doing and take a taxi to the ferry, ferry to taxi in St. John to fix my car. i just feel better knowing Delbert and Elaine will send help." (Referring to Delbert Hill, www.delberthillrentals.com).
This is a very good point. It's why we always rented on St. John and now keep a car on island (a nearly 15-year-old Suzuki Sidekick which is dwarfed by all the SUVs which are too large for the island's roads. You're not in commuter traffic on expressways on island! Do St. John a favor - no matter what, rent small. It's a small island.
The message thread about car rentals began in May when a visitor from Oregon complained about being treated rudely by an agent working for Conrad Sutton, www.conradcars.com. But another vacationer, "Josieann", said she'd used Sutton in April, "and had absolutely no problems. The person inside (I think it was Conrad's daughter) couldn't have been nicer."