Hi there Bram,
No one seems to have addressed the issue of your first nights hotel cost: is it already paid for on San Pedro as part of your trip, or are you buying the first night wherever you decide to stay?
It will cost you $25 us for a cab downtown, then $10 us in the morning for the water taxi, so about a $20 "saving".
What is your room rate in San Pedro? The Belcove,
www.belcove.com is right by the swing bridge, is $27.50 a night for a budget room, and they do a $20 airport pick up, but if you want to do anything other than stay in your room and relax after the trip, downtown Belize City may not be the experience that you are looking for!!
If I'd just spent $1300 lunies on a flight, I'd get to Sun Up for a combo fried rice ASAP, and forget about trying to save $20!!