I think on one flight in the last couple of years the seat cost was actually less than the APD - so for the two under-16s, the *seat* cost was £0.00.
But of course we needed suitcases for the gubbins required for a week or so ("do you *really* need five books for a week-long stay?").
But surely the point is that the prices are set by the algorithm. And whether through machine learning or human intervention I *suspect* that they are pursuing the retail model of a low "headline" price followed up with hugely profitable "accessories" - a bit like buying a TV and games console at a great price - and then being sold an extended warranty, or an HDMI cable for about three times what it should be.