On most islands, a taxi driver giving tours quotes a price per person but it is based on filling their 8/10 passenger van. You may find yourself sitting in a van waiting for him to scrounge up more fares and your small little group may not be compatible or wanting to go to the same spots.
However, instead of paying $20/$25 per person for a taxi, we have had great success negotiating a total price of $50/$100 for the "tour for two" and stopping where we want to go (usually for a photo opportunity). The extra money is usually well worth it if we want an exclusive ride to our liking.
Taxis are usually moderated on most islands in a rotation system so a driver who may not be at the head of the line today will gladly accept this type of a fare.