The island is Saba in the Netherlands Antilles. It is served by Winair (Windward Islands Airways) from St Maarten. Needless to say its pilots don't go to Saba without check rides.
Winair's stage lengths are very short - 15 minutes or so. A couple of years ago they retired a Twotter that had flown well over 100,000 cycles (IIRC).
The Twotter may be expensive because of its abilities and two engines. You seldom get more than you pay for in products.
Winair also has some Islanders, which are smaller than your request. Maybe the Trislander would satisfy you, but I don't know much about this sort of thing.