I didn't want this to get lost in the comments. Here is what a dragonhonr bow does in my DF game, Felltower:
"Horns can be sliced up into strips and used as fronting for a dragon-slaying composite bow. You'd need to pay the construction cost for the bow (at least fine quality, as well) and the enchantment cost, but if you got it up to +3 Puissance and Accuracy (even if vs. Dragons only) it would get the "slaying" quality - any crit is max damage, any 3 is 3x max damage. One bow/horn."
"Dragons only" as a Puissance limitation would be the Bane enchantment ($2000) and reduce the cost of Puissance in half. So Puissance +3, Dragons only, on an otherwise Puissance +1 bow would be $90,000 (10,000 energy, divided by 2, minus 500, times $20.)
As I said to my players, "This may seem stingy, but these things are not available for sale, anywhere, so it's a gateway to a special magic item." And it is. Don't expect to just find dragon slaying or dragon horn bows laying around. Want one? Kill a dragon.