Primas is one of my favorite places, but it is not cheap. They make their own pasta and the seafood is VERY fresh. I also think the grilled lobster is the best on the island. Dinner for two with drinks will break 50 bucks if you're hungry.
My recommendation for a cheap but good place is LaMission, not the one by the square and not the one near the airport. There is a third LaMission on 30th. if you can get the cabbie to take you there. It is where the locals go. I had filet of grouper,baked potato, steamed veggies, flour tortillas, refried black beans and guacamole for 60 pesos and it was VERY good. My wife had the " Big Boy Gringo Mexican Platter " which was loaded with enchiladas,rellenos, tacos you name it and couldn't eat it all for 70 pesos. I highly recommend this place. I'll be there in a couple of weeks.