I assume your prices are in RDS.
Why don?t you contact Joanne at Castilla/Sams Bar.
Right downtown, not very fancy, but all included.
She has only 1 apt, the rest are rooms, but I think it goes for around 5000-6000 RDS per month. Check if it is available.
Her username on DR1 is samsbar, send her a pm.
Try Villa Serena on Calle Villa Neueva in Puerto Plata. The number is 809-307-5226. It is an apartment-hotel, renting efficiency rooms. All included in that price range. The owner's name is Bruno. He speaks Spanish and Italian.