Dragons are hard to kill. This really shouldn't surprise anybody. I mean, they're giant armoured flying lizards. Clearly any prospective dragon-slayer, or for that matter, any king with a large treasury and a smidgeon of healthy paranoia, will want something to use as an equalizer in a prospective battle.
Setting aside converted siege weapons, asbestos-coated armour and the rare wizard both powerful enough to battle a dragon and crazy enough to try, the best equalizer are magically prepared arrows of dragonslaying.
Admittedly, the name is a bit of a misnomer, as they can't actually kill a dragon except by a truly lucky shot, but unlike normal arrows they entirely ignore the dragon's ridiculously tough hide, and cause debilitating pain quite out of proportion to the amount of damage they inflict. These features are what they're sought for. Being able to hit a dragon's armour and still cause it to fall out of the sky from sheer shock is a big help.
Perhaps ironically, they're quite rare due to the fact they're made out of dragon parts.
Before the magic spell that imbues the arrows with their power can be cast, the heads of the arrows have to be made from dragon claws (in case you were wondering, a given dragon has from six to twenty-two claws, depending on its physical layout).
The next stage, once the heads have been made and affixed to a suitable shaft is to soak the arrows in the still-warm blood of a recently killed dragon.
Because the claws are even harder than the dragon's hide and filing them down to the proper shape can take days, and the blood cannot be more than half an hour old, for every quiver of arrows, two dragons have to slain.
Which explains why they're so amazingly expensive.