Recursive SQL (DB2):
(WITH X defines a temporary view)

WITH trips(destination, route, totalcost) AS
 ((SELECT destination, destination, cost
   FROM flights
   WHERE origin = "SFO")
  UNION ALL
   (SELECT f.destination, 
           t.route || ',' || f.destination,
           t.totalcost * f.cost
    FROM trips t, flights f
    WHERE t.destination = f.origin))
SELECT route, totalcost
FROM trips
WHERE destination = "JFK";

NB: There is no stop condition, so this will run indefinitely..