The flight from Corozal to San Pedro is US$47.50 one way, US$90.50 round trip.
I don't see the cost on the Thunderbolt Water Taxi website, but it only leaves at 7am and 3 pm.
How you get to where you're staying in San Pedro, depends on where you're staying. If you fly on Tropic and are staying right in town, it's possible to walk. South of town a regular cab. North of town many places provide transportation, or a cab to the water taxi dock, then a boat ride.