Many departure taxes and charges are fixed price items NOT a percentage of the fare.
Most of low cost carriers would prefer to seperate out what they call "the cost of the flight" from what they call "taxes and other charges". They claim this is so you can see how cheap their fares are compared to the high rate of taxes and other stuff "outside their control". At least that's their claim.
I note that on the Ryan air web site they now advertise their special £10 flight offers as "including taxes, fees and charges" so that's a step in the right direction I guess.
I once paid 99p for the ticket and about £20 in taxes and other charges but then I knew their 99p advert didn't include tax and charges so no shock horror.