Sorry, there's a wide range of costs depending on which tour you go with and when you go and exactly where you go and how many people go ... You might want to visit www.belizex.com
and if it's important to you then invest a few dollars in telephone calls to Belize. Or buy a good guidebook to Belize, though not all tour costs will be any one guide.
Frankly, if costs are a main consideration, I'd just stay at Martha's in San Ignacio or Aguada nearby in Santa Elena and take tours offered by folks in town. You'll find plenty of options.
Guide to Mainland Belize (Amazon.com)
Fodor's Belize & Guatemala Guide (any bookstore)