I've just returned to San Pedro from a week in Placencia. The whale shark season there has pretty well ended, and while it's quite a nice place I wouldn't choose to go to Placencia for general diving - trips to the reef are very long and commensurately expensive. What do you anticipate doing in Placencia? It currently has no real budget accommodation.
Unless you have specific reasons for visiting Placencia, I think you might do better diving from the northern cayes (Ambergris & Caulker) where the reef is close and diving is relatively cheap, and for a change going inland to San Ignacio (lots of cheap accommodation) to see some jungle/Mayan ruins. And because of physiology considerations I'd dive before inland pursuits. A Blue Hole trip is cheapest made from Caye Caulker.
Air prices are generally much higher out of Belize City than out of Cancun, and one way tickets generally cost much more than two singles. I'd expect the cheapest route to be flying in/out of Cancun
Unless you have specific reasons for visiting Placencia, I think you might do better diving from the northern cayes (Ambergris & Caulker) where the reef is close and diving is relatively cheap, and for a change going inland to San Ignacio (lots of cheap accommodation) to see some jungle/Mayan ruins. And because of physiology considerations I'd dive before inland pursuits. A Blue Hole trip is cheapest made from Caye Caulker.
Air prices are generally much higher out of Belize City than out of Cancun, and one way tickets generally cost much more than two singles. I'd expect the cheapest route to be flying in/out of Cancun