Well theres regular dragons, brutal dragons and mithril dragons. So I decided to make a suggestion about frost dragons. Burying dragon bones gives about 600+ XP, burying frost dragon bones is about 825 XP and its harder to get these. I think that frost dragons are stronger than mithril dragon, so frost dragon bones will cost more than 10k XD. He will drop visage, some dragon items, shield halfs, dragon arrows, some kind of herbs, gems, ores (coal, runite?), half keys, some cash piles (15k-30k) etc... . I dont know that what dungeon are these dragons, so its up to you that where they spawn.
Frost dragon looks like this:
, Thanks
0wned man
ADDED: I found a dungeon: Asgarnian Ice Dungeon