Andrea,
Those are dominicans, not gringos. They know their own country, know how to get the best deals, tend not to live in Gringo preferred neighborhoods, and don't have to acclimatize themselves to living under third world conditions.
Most gringos tend to come because they think they can get a better quality of life than where they came from for less money. The reality is, for the first several months or years, they usually wind up spending more due to where they choose to live and having to pay the Gringo Tax on just about everything.
Please don't mislead the OP. If he shuts down his life in his hometown and tries to make a go on $150US, he will fail. Miserably. I don't mean to be Mr. Gloom-an'-Doom, but clearly he hasn't been to the island enough and done enough hands-on research, or he'd know the answer to his question.
To the OP, there have been a number of recent threads on this topic. From what I read, if you're by yourself and don't intend to live a lavish lifestyle, you should be fine in STI or SD on about two grand a month to start. As you get to know your way around, you might be able to shave a bit - but not too much - off that number.
Good luck to you.