Hey Snuffy,
It doesn't really rule out the Sosua/Cabarete areas.
It depends a lot on what he means by big, and is that price, bills included, and what are his requirements, like 24 hour power, pool, etc.
As soon as you get out of town, prices drop substantially.
Does he intend on having wheels?
Does he prefer to be isolated and away from town?
Is he hoping to be right on the water?
There are so many factors to take into account.
You could luck out and find a house for that price that would normally be 30,000 RD per month, but the owner is just looking to have someone reliable live in it and take care of it as they are not using it, or have it up for sale.
It's just a matter of taking the time to shop around & talk to the local expats, to pick up clues, as to where to look.