A single-use 13th caster level Disintegrate item would cost 390 IM.
It would inflict 26d6 damage if the target failed a DC 19 Fortitude save, or 5d6 damage if they make the save.
Since Amrelath has a +20 Fortitude bonus, he would only have failed on a Nat 1 and even if the Disintegrate was Maximized, which would have required it to need a minimum caster level of 17th, it would only inflict 156 damage. That's a lot, but Amrelath has 274 HP. At an average of 91 damage per 26d6 arrow, and assuming the arrow itself also inflicts damage, he could have tanked a couple of them before it got dangerous. And Wings of Cover would have been good for blocking at least one.
All in all, a fantastic weapon to use against most beings, but not something with the tankiness of a Mature Red Dragon. If they set out to assassinate a Dragon, using the Disintegrate arrows wasn't a good option. Would have been better for
@DragonParadox to keep the Slay Living arrows and simply have them be ineffective.