A lot also depends on when you arrive. You might have to stay a night somewhere along the way.
1. Water taxi Chetumal to Ambergris: BZ$70 per person, one way; BZ$120 round trip.
There are water taxis between Ambergris & Caye Caulker; the last one departs at 4:30 pm. Cost: BZ$15 (maybe BZ$10 ?).
2. Premier Express bus from Chetumal to Belize City: BZ$16 per person, one way (3 hours). Other non-express buses are about BZ$12 or $13. Get the express; you'll thank me! If you get a non-express bus, you might want to spend 20 pesos to get to the Nuevo Mercado terminal from the ADO terminal.
In Belize City, you can walk from the bus terminal to the water taxi terminal, but if you get a taxi, it will cost BZ$7.00 per taxi.
If you get the Linea Dorada bus from Chetumal to BC, that will cost BZ$20 per person, but it will drop you at the water taxi terminal.
Water taxi BC to CC: BZ$15.00 per person, one way; BZ$25 or $30 per person, round trip (depends on the water taxi company).
Have a look at my BelizeBus site; it has more detail, and links to the water taxi companies:http://belizebus.wordpress.com/