One time in Cosumel, DD, DGD, and I took a private tour. The tour guide was awesome. He knew his stuff and spent several hours taking us everywhere we wanted to go. It was a wonderful trip. At the end, we did tip him, and tip him, we don't know how much we gave but I'd guess it was $50-$60 over the fee of our tour. (We didn't have any more money with us, because we spent it! lol)
The next day was Grand Cayman, we tipped the tour guide (I think) $10. He was okay.
It really matters how you enjoy the tour!