Its really not.
I'm not talking about in game points comparisons.
The Destrier isn't much smaller than a Questoris, and has improvements compared to the Questoris kit. Knights kits are very expensive as a whole of course, argue they are all collectively hilariously overpriced if you want, but the cost relationships between Destrier/Questoris/Castellan/Cerastus doesn't look wrong to me in terms of model content.
As for the Defiler, when the model is constructed it looks quite small and unimpressive for an almost £90 rrp kit. Its perception based, does the kit feel like its worth it when its constructed.