Last time we travelled to St Kitts from the U.S. East Coast, flights were around $750 with a connection in Miami through AA. This was in early August (mid season), so not sure it helps. I also thought it was very high, while we did eventually pay it as I couldn't find anything cheaper. We had a great last minute II exchange, so felt that made up for some of it. We were not a fan of St Kitts Beach Club and would not likely go back. The villa was huge, but the resort just didn't hit the mark for us. There are many other beachfront MVC resorts we like far better.
We had an II confirmation for St. Thomas this past April. Airfare was about $800 per person. We ended up cancelling and choosing to go somewhere else because of several factors (including airfare cost). We haven't been to Frenchman's Cove yet, but hear it is very nice. I'm certain we'll exchange there in the future.
I think Feb and March are peak season for both locations, as such, it would not surprise me if you had to pay a premium to fly. I would suggest watching flights for several weeks (or even a month or two) to get an idea of approximate costs until one of your exchanges is matched. You'll then be able to get a feel for the average prices to fly as well as the high/low points prior to your exchange being matched. Google Flight tracker works well, among others.
The Marriott Aruba properties are also very nice, if you want Caribbean.
In reality, no flights are cheap these days. Sometimes, we just have to pay the price, forget we did, and enjoy the vacation!