$11.
Let's be honest, nobody likes getting "Nickel and Dimed". Small, incremental increases do add up, but for me at least, $11 is not likely going to affect my "Go/No Go Decision". If the charges continue to add up, things will likely change, but for now, 11 bucks added on to the cost of a trip is not going to change my mind one way or the other.
Everyone's threshold for when it reaches their personal Go/No Go Point will be different, but as long as the tax is in place, if ya want to go to Cozumel, then I'm afraid you're going to have to, as someone once said: "Suck it up Buttercup."