A lot depends on where you are traveling and whether or not you are transiting or stopping at a location. I just purchased at 28 segment AONE5 for about US$5,800 and ended up paying about US$800 in taxes. This does not include those taxes which are collected locally, not as part of your ticket (e.g. UIO, BKK, AKL,...).

Here are some averages for stopovers from recent trips but they can vary widely in some locations: Asia - US$15; South America - US$30; US domestic - US$10; US arrival - US$28; SWP - US$30; Europe - US$15 except LHR (one of the priciest places to stop) - US$75.

Make sure you have the taxes itemized on your ticket or in each airline's PNR that is generated, otherwise you will likely end up paying them twice.