Taxi from PGIA to Belize City: BZ$50 per taxi (not per passenger); 15 to 25 minutes depending on traffic.
Haven't yet seen a taxi with a meter or a rate chart (though I think there are standard rates).
Trips within Belize City are BZ$5 to $7.
What you have to make clear is the currency -- if the driver says "5 dollars", make sure you both understand that it's 5 dollars Belize, otherwise at the end of the trip, the driver might says that he meant US$.
Shuttle: BZ$40 (check the website)http://www.belizeshuttlesandtransfers.com/Belize-City-Shuttles.php