Difficult understanding the purchasing power of a peso

First welcome to the forums. If you are in a tourist area then you will see this constantly. There are two or more prices based on how the locals see you.

If you shop in  bigger stores where there is no bargaining then the price is the price.  Its is reality in tourist areas.

If you know the language and  are  clear on what the prices should be then it gets easier.

Learn to negotiate. Learn to  find the best places to shop.   Ask  here about pricing,  many of us have a lot of experience and time here.

Interesting topic. It can be difficult to negotiate with locals that see you as "wealthy", or "privileged", which I don't view myself as, but I understand their view. So it's either their way (price) or the highway in some instances. But I make the attempt.
Some expats have a problem with what amounts to subsidizing some of the goods. So, a banana may be 10 pesos for a local and fifteen for me. That doesn't really bother me I guess. I have other things to worry about. What does irk me is the blatant rip offs, or the get-the-gringo attitude. The walk around street vendors are especially bad at this in LT, since they primarily serve the tourists I guess.
I have found attitudes of the vendors here very individual. Take it or leave it, or let's come to an agreement. Generally easy to discern. So, let your feet do the talking in some cases.

I have found a  polite but strong no  usually goes a long way. My experience is once they get used to seeing you and understand you live here the attitude will change. But of course not always.

The ones that apply hard pressure never get my business!

And for those who are tourists and want to  get local prices, well  unless it is a blatant rip off, get over it. You are a tourist.  You will not get local prices  often!   You are not a local so you dont get what they get! Simple.  Once you live here you will get a  version of local prices! 

This is common in almost all  areas that rely on tourism. Go to downtown NY and see tourist prices. Go to Toronto or Montreal or Victoria. I guarantee  you will find tourism pricing!

I try not to shop in places that don't display prices! At the grocery store, I know that everyone who shops there is paying the same price because it's right there on the shelf and programmed into their POS system! At the liquor store down the street (Ron Factory), there are no displayed prices - you pay whatever the person working there at the time thinks you should/will pay.

I've also had someone lower a price after seeing my cedula. Whether it was a courtesy to me as a resident, or if he realized I wasn't just on vacation and could be a repeat customer, I'll never know. The first time, it wasn't intentional, but I now make a point of having it visible in my wallet.

I've also had the price magically change to however much I had in my pocket! For this reason, I always keep smaller bills and coins in a different area of my wallet.

So true.

Outside tourist zones, you get to understand actual prices and if asked to pay more walk away. They soon react. The other day I saw a truck selling face masks. Stopped and asked the price for a box of 50. He looked at me and said 1500 pesos. I walked away back to my car ready to drive off. Knock on the window. How much? I said 1100 pesos. Done deal. Should have offered less but that was less than I buy at the farmacia so I thought I had done well.

Also, away from resort areas you should ask for a discount. It is always there for Dominicans so do as they do. Of course if you cant negotiate in Spanish it isnt going to happen.

I know most of the businesses I buy from now and they know me.