I don't think St. John is as bad as Atlantis but many of the Caribbean locations can be quite expensive including Cayman's, Antigua, Aruba, St. Marteen/Martin and the like.
The Westin St. John (which per many reviews is over rated, the beach there is poor and the time share units aren't near the water) is pretty much impossible to get through a DVC-II trade. Ownership there is very expensive (much more so than DVC), so much so that owners rarely put any time up for trades.
I don't think too much of the Westin on St. John. I have stayed in the hotel (not through II) and visited the timeshare section. All of the timeshares are across a street and most are up on a hill. The beach at the Westin is one of the worst on St. John (It's hard to believe there is such a thing but this is it). The water is always cloudy and it is in a protected harbor with MANY boats. This may sound good but keep in mind that the protected harbors in the Caribbean have lots of boats in them and they dump waste directly into the ocean (it is ground up first for the most part, but not cleaned in any way). You can not snorkel at this beach as it is always so cloudy.
If you go to the Westin and are a beach person, you will want to head over to Caneel Bay or to the other side of the island every day.