Nancy,
How "near" SD will you be? There are several "American style" supermarkets and grocery stoes, but they're all in SD or Santiago (can anyone tell me whether Puerto Plata has finally gotten one?). These are mostly chains like Nacional, Pola and La Cadena, although there are some good smaller, more grocery store-type ones that are not as easy to find but are better priced and usually have better produce.
If your resort is out near Boca Chica or Juan Dolio, you may have to settle for a colmado. If there is a supermarket in Boca, I'm betting Jim Hinsch, a board regular, knows where it is and what prices are like (how about it, Jim?).
General rule about food prices the DR is, any fresh fruits or vegetables are much cheaper than in the US or Canada, especially if you buy them at the market ("Feria"). Anything canned or prepackaged, prices are on a par with those in most major American cities (take from someone who did the family grocery shopping for 4 years in Santo Domingo!).
Hope this helps.
Best Regards, Keith