There are several ferries/water taxis to Belize City. I think the Caye Caulker Water Taxi Association boats (leaving from the old Shark's dock) are the most dependable. They cost US$12.50 one-way and take maybe 1 1/4 hours, depending on whether they stop at Caye Caulker and/or Caye Chapel.
Regular Belikin in bars costs from US$1.50 to $3 or so, depending on the bar's philosophy. Premium Belikin is sold by some of the fancier places and is a bit more, US$3 to $4.