I am a bit of a shopaholic and I have done quite a lot of analysis on Pricesmart prices. Some of their things are very cheap. Apple computers are way cheaper than you can get them in Mac dealers in Santo Domingo. Some items they have in stock, some they have to import, but the prices are good. But things like orange juice, wine, Gatorade, all are about the same or more than Jumbo or Sirena. They often come in with good prices on US imported items like Pancake mix. I go to the branch in Santo Domingo and you cannot get in without showing a membership card.
Carrefour has got some things you can't get elsewhere (their croissants are nicer than Nacional in my opinion), their champagne range is excellent and good value. And some strange things like shower gel are much better and cheaper than anywhere else. But you have to be careful, some of their stuff is way more expensive than other supermarkets.
One thing I have learnt is to scan the daily papers, supermarkets really do have some amazing specials and these only last a day. Plaza Lama often has really cheap beer and rum. Nacional also has some excellent specials. If you shop around, you can save a lot.