There is no shortage of patients for plastic surgery, and from what I've seen, it is mostly locals. There are some foreigners, but I would say that percentage wise, it is less than 25%.
For foreigners, there are better options than DR in terms of pricing and quality. Example: a breast enlargement procedure in DR costs $3,000 USD + the cost of medications and follow-up care. For the same $3,000 USD, you get transportation, and a week in an aftercare house, including nurses, medications, etc., in Colombia. Colombian plastic surgeons and hospitals are better regarded than their Dominican counterparts.