The only public archeological area near Dangriga that I can think of is Mayflower. It's not one of the major Maya sites. There are some ruins in the Cockscomb Preserve but these would not be on a tour. Some tour operators in Hopkins and Placencia also run trips down to Nim Il Punit and Lubaantun near PG.
How much it should cost would depend on where you are going and what is included.
You can also visit these on your own, if you have a car. There's a visitors center at Nim Il Punit and caretakers at all of the sites.
Yes, all tour guides in Belize have to go through a training program and have to be licensed.
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