Some of the "hotels" in Gazcue do monthly rates etc. In fact I am "living" at Hotel Residencia and have my DSL installed.
Mona Lisa does monthly rentals but am not sure of the pricing.
Hotel residencia the pricing ranges from 12,000 pesos to $600 for the month. It is a bit complicated as the hotel has 3 owners. Each owner has their own rooms and determines their own rates. For the "nicer" rooms with AC and Cable you can call 809 412 7298 or
www.hotelresidencia.com to speak to the owner that concentrates more on monthly rentals in more "basic" rooms you will have to PM me and I can make a phone call to get his number.
Hotel Danae also on the same street also does monthly rentals. Prices start at about 14,000 pesos and go up to about 20,000 pesos. They just added new rooms on the second floor. I have not stayed in the new rooms yet.
www.hoteldanae.com phone number is 809 238 5609. They have a receptionist named susan that speaks english.
Gran Mansion also on the same street, I believe does monthly rentals but I have never stayed there. It is literally 2-3 doors down from Hotel Danae. I believe Ken has stayed there in the past and may have more info.
You will find that in many cases paying the monthly rate pays off even if only staying 3 weeks.