If you go, for example, to the BMI website, you will see on the homepage an electronic billboard offering flights to Amsterdam, Dublin, etc from £25 *including fees and charges*
If you go into the booking engine, you will see these flights listed, from £7 *excluding fees and charges*. It's only when you have selected the flights will it tell you how much you owe.
If they can advertise net fares on their home page, why on earth can't they put net fares into the booking engine?