Several places you can stay that will cost you around RD$400/night. For an extended stay, you might be able to negotiate a slightly better rate.
Hotel La Danae on Calle Danae: tel 238-5609/5591
Hotel El Senorial: tel 687-4367, fax 687-0600
Hotel La Grand Mansion on Danae: tel 682-2033/689-8758
I usually stay at La Grand Mansion. one person for one night is RD$350. Hotel La Danae is only a few doors away. It is new. My wife says she will stay there next time we go. A plus for you would be the presence of a small refrig in each room. It only has a few rooms available at this time, so if you plan to stay there, reserve as soon as you can.
Danae is a quiet, safe street.