Taxes are unavoidable, and they change depending on the carrier, amongst others it may depend on which handler the company use (handling charges), the airport they fly to (landing fees), if the aircraft has an entertainment system which will warrent a fuel surcharge as the plane will be heavier.
A flight with a budget carrier may originaly cost 99p, in which case it would be silly not to expect the taxes to be less than this.
On the other hand, there is the "Administration Fee" or something like it which seems to me like a profit booster
I think advertising prices without taxes is fine, providing it is clear that taxes will be added before you commit yourself.
WoolyAlan