Still that's $650 from US to Belize City, which is cheaper then some fares I've seen.
You'll have to buy the $110 bze-San Pedro ticket regardless of where you fly from.
But you're right it'll probably be a wash, unless you can get a good special price into Cancun, or if that Cancun-Belize City flight is cheaper. I've taken a few commuter flights around Latin America that we're surprisingly cheap though, so we can hope. I flew Flores-Guatemala City for $200 RT, but that was a few years back.