At Chankanaab in January we paid $10 each for admission (including 9yo DS) and DD aged 6 was free. We took the taxis at the back of the shopping mall as you get off the ship, and the standard price was $10 one way. There were lots of taxis waiting at Chankanaab to bring us back, too. Make sure that ALL the seats in the taxi have seatbelts - not just the front seats!
We took bottled water, antibacterial wipes, and snacks with us, and after using the bathroom facilities (basic as opposed to awful) were pleased that we'd done so. We also took our own snorkelling equipment, but rentals were available too. There were lots of lounge chairs on the beach and casual outdoor restaurant facilities if you wanted them. The children were thrilled that we were able to see a couple of ignuanas sunning on the rocks, too. Including taxi tips, we paid $55 for the excursion for the four of us. I may not remember the prices exactly, but it seems to me that the Disney 1/2 day snorkelling excursion was going to cost us $45 each per adult and $35 each per child for a total of $160 (including rental of snorkelling equipment).
Hope this helps!