It is a buyer's market. There is plenty of room to negotiate. If you buy directly from the painter, keep in mind that if he gives it to one of the shops to sell that they will take at least 50%
Whether buying from the painter or from a shop, it pays to take your time, and walking away is a good strategy.
I started by saying you can't generalize because there is a lot of crap being sold and there is also some very good stuff. Most of the paintings we have were bought from a dealer in Santo Domingo, but we have purchased 3 on the beach. They are very well done and unique, The smallest is 25"x18" and we paid the painter RD$900. The one 30"x40" we paid RD$1500 or so. It is my favorite of all the paintings we have. The one 23"x31" we paid RD$2000 or RD$2500 and have been offered more than that for the painting.
I think that is all some of those mass produced paintings sold at the beach by every shop are worth, but the artists probably have more than the amount you offered invested in canvas and ink, not to mention their time.
Keep in mind, this is still the slow season, I was able to buy FOUR of those Mona Lisa's for the original asking price. As Ken stated, walk away a few times, even come back the next day. The asking price can be 5X times what you should pay with some of these guys. Your RD500 Bid for two as not that far off!