In country side you can live cheap - $600-800US a month per person if not less.
Just by all means avoid as much as possible those brutally overpriced tourist (and baby-boomers) traps like Santo Domingo, Sosua, Cabarete and Punta Cana where even a very basic lunch for two in Baley`s for instance will cost you $30US or more. (No bad feeling about Baleys, just an example of unbelievably inflated Sosua prices oriented on dummies with back packs).
Here you`ll need $3000 a month per person just to barely survive.
So again, all depends on "location, location, location" ( Juan P.)