the taxes vary so much on international flights, what determines them?

Answer 1

Much like making international phone calls rates can vary widely based on where you are calling and or flying.

The country of termination can determine their own tarrifs for flying into each country, which varry the cost of tickets greatly.

As an example some international calls can be made for as little as 10 cents a minute, other countries it can be well over 10 dollars a minute.

It is all about the taxes.