Can't remember the exact price, but very cheap - maybe $20 tops.
They offer scuba, too, even if your not certified. Lots of interesting coral and colorful fish - and they will take you into a cave if you want! A little difficult getting in and out of the water on some steps, but a lovely experience.
The price of the equipment depends on what you use, for everything we needed, including mask, fins, and life vest, it cost $9. It also costs $10 to get into the park. Overall we were disapointed with the snorkeling experience here as the waters were choppy and its was fairly difficult for the kids to get into the water. If I was to do it again, I would try another beach. There was a few fish, but they weren't very colorful.
I agree with the opinion about the choppy waters at Chankanaab. I was there last week (Wed. the 18th), and I heard a number of people saying that they were getting pushed into the rocks along the shore while in the water. Maybe it was just a windy day, but people were having trouble. Also, someone told me that the views underwater were not as good as she had hoped.