Cape Air has suspended flights from SJU to NEV so only flight options from SJU are Seaborne and Tradewind. Prices vary depending on time of year and day. Seaborne is usually less than Tradewind. Go to respective websites for actual pricing. Winair is only flight option from SXM.

Whether you find any flight prices "reasonable" depends on your definition of that word. It is rarely inexpensive to fly between islands in the Caribbean.

Smoothness of ferry ride depends on sea conditions the day you travel. On waiver days the ferry will alter its course and hug the shore lines of both islands to minimize wave action. It will add about 5 to 10 minutes to the trip but make it a bit smoother.