If I remember right it is $35bz to GC. That includes bridge toll. It is regulated by the town and the taxi driver should have the rate schedule with him and you can ask for it. The rates increase has you head North.
If you email or call Grand Caribe they will let you know the fare. Although I haven't used a taxi to go there I was talking to a taxi driver parked at the Palapa Bar and I asked him exactly that question. He pulled out a schedule. I thought that there was one on line that I saved but maybe I only printed it and left it in Belize. Have a good trip.