Prices vary wildly, depending on demand. Cape Air has flights three days/week from STT to EIS. Last minute tickets run about $100/each, but advance purchase can be much more. Another good option is chartering a plane, which is cost effective if you have four or more people.

From San Juan, Seaborne and Cape Air are two popular options, and they codeshare with several US airlines. Cape Air has commuter books, which could save you money.