Here is a link for Taxi Fair estimator. During our 2010 trip we took a Mears bus service it picked us up at Pop and brought us back round trip was 18.00 a person I think.
Bus is the cheapest option! $1.75 one way. shuttle is $16, and car rentals per day cost the same as taxis, both are $30 plus tax and the tip incase of using the taxis!