While I don't have any specifics for PDP (as I've never been), generally, my South America "hire a driver" experiences have been decent.

(1) South of Chile - had the little hotel call someone and set a price - it was fair. (This is someplace I've already driven around on a few occasions.)
(2) Mendoza, Argentina - had the hotel call someone and set a price - it was fair.
(3) Colonia, Uruguay - we hailed someone on the street to tour us around after we did all the walking we could - fixed a price - it was fair.

Given the generally high costs of car rental, the cost of fuel, and the stress of driving on unknown (and often not-well-marked) roads, not only is the driver worth a small surcharge over a rental car, but you might learn something new, too. If you weren't really planning on driving the day you arrive, you might want to keep your car reservation, check on alternate arrangements when you arrive, and then decide whether you want to cancel your car reservation.