Anything from about RD$1200 to RD$2000-3000+++ depending on the carrier (Orange vs. Verizon/Codetel) and how fancy-pants you think a phone has to be to do its job, e.g. play MP3 files and take photos or not. (Hint: it needs only to send and receive calls). Orange seem to be more prevalent and probably cheaper, though I still use Codetel as I think the signal is far better. They use cards, good for 50-70 days and after the card dies due to use or expires due to the passage of time, the phone will still recieve calls for another 2-3 months.
NB: Other than sankies, posts about phones, bringing phones, activating phones, renting phones etc., are about the next most common and a search will (and would have) help.