Try ropa de paca...
If you are looking for cheap plus sized clothes, and have enough time and patience, it's worth finding a local person who sells clothes from a paca, those giant packs of used clothing brought in from Haiti. Some people sell from their house, others from the market places. These people are usually located in the barrios. But ask around, everyone knows someone that sells them.
Most of the clothes in there come from the U.S., so there are a lot of plus sizes. You have to be willing to dig through the piles of clothing to find a good item and not be too picky as far as color/style. Sometimes you can get something nearly brand new and name brand!
If you are going to be around a while and in the market for those clothes, it's worth getting to know someone that sells paca clothes, because they can pre-sort the sizes you are looking for out for you, assuming you are willing to buy and pay a slightly higher price.
I'm not sure if the paca ladies charge more for plused sized clothes, since they tend to charge by quality of item but also take weight of the article into account (because they buy clothes by the pound and need to turn a profit). Prices for paca clothes for me have ranged from 20 pesos to 100 an item.