Unless you're a masochist, by far the easiest and best way between San Pedro and Coro is to fly -- there are about five flights a day each on Maya Island and Tropic Air, 25 minutes, US$37 or so.
From Coro to Belize City, the only real option (other than drive yourself, which is best) is Novelo's/Northern Transport -- the express is not too bad and only a little over two hours, though the local will take about 3 hours. There are no flights from Coro to Belize City except through San Pedro. I'd be surprised if you could find a taxi for less than US$100.
--Lan Sluder