We had to pay a departure tax when we left Jamaica after our wedding. And they want cash. I think I also paid one in South Africa, and if I remember correctly, Vancouver BC had a departure tax that went to the airport.
Here are some departure taxes in the Caribbean:
Antigua and Barbuda: $20; Aruba: $36.50;Bahamas: $15;Barbados: $27.50;Belize: $39;Bonaire: $32;Cayman Islands: $20;Bermuda: $29;Dominica: $20;Dominican Republic: $20;Grenada: $13;Haiti: $30;Jamaica: $20;St. Kitts and Nevis: $20;St. Lucia: $26
So, I guess the $20 for the US is not bad, but on the other hand, these small islands are not on par with the financial strength of the US, so it is a bit like comparing apples and oranges.