My wife had her medical done by the doctor in Puerto Plata, very straight forward, professional, the doctor speaks english and the waiting time was not excessive. I do not remember the cost offhand but if you are applying for a Canadian residence visa the cost is listed on the website. Blood, x-ray, etc analysis is done in Trinidad.
The police certificate requirement to present in person in Santo Domingo is new since we applied in 2009. You may want to make this a new tread so future applicants can follow it. I would but I don't have permission to post a thread.