hello jenb,
since I come from Ontario and now live in Jamaica I can easily say that most of the local things that grow here will be a little cheaper, but not much... if in season like mangoes, for sure u will get a lot more here for yr money compared to Ontario prices.
BUT.... always try to go to the big open markets, there is where u get the best price and quality...buying inside a supermarket is not the best idea... IMO.
there is a great farmer's market in Negril at the Beach Park every last Wednesday of the month, and the Sav market is great too.
since u r coming to stay for a longer time.... please try to come by the Catholic Church or St. Anthony's Kitchen
blessings,
St. Anthony's Kitchen