Everywhere in the world things like this go on, the aggravating thing is the extreme of the DR, and that a deal is never a deal. In the US if you walk in to a car dealership the first question they will ask you is how much can you spend. My response is as little as possible or best value for the money. I'm not there to spend every penny I have to give it to you. The problem with the DR is everyone is like that car dealership and want all your savings for whatever.
For a photocopy or print out gringo price could be 10 pesos, while 3 pesos to locals. That is extreme, but you can live with that, but that is way higher than American prices. But when you get into the taxi situation, you might as well buy a car. Something that is 300 pesos to a local can be 2000 pesos to a tourist, taking that safe hotel taxi (same guy, same car). Then they will even complain that you are ripping them off for that amount and it should be more.
This makes the DR not a cheap place.
I could make a comparison to Las Vegas these days. People that go and visit and stay on the strip think it is one of the most expensive cities in the world, but if you are local and get off the strip, it is one of the cheapest. Key there is if you are local you then can get the deals, in the DR, it'll never happen, or you can live your life through middle men every time you need to get something at the store, and hope that middle man isn't taking a cut anyway.
I think it is the extreme of the DR that gets so annoying, and really them not caring if they get a good deal, but whether they have one the battle. The goal is for them to be told by others that they pulled one over on the sucker gringo more than any money they get. Also the math skills aren't that great. Often you will find buying in smaller quantities is a discount from the bigger packages, or my favorite, buying 2 half chickens is cheaper than buying 1 whole chicken. They just don't understand the basics of a deal, but boy do they love negotiating.