Prayer is an expensive 99 stat, this guide is short for now until I get time to expand on it.
Dragon Bones - I recommend you use double XP to get 11,520XP per bone on the altar
Frost Dragon Bones - They grant you 28,880 XP each with Double Experience.
Use your bones on the altar to get 2X XP as burying them.
Those are your two options. They range 100-200k each.
The fastest altar is not the one outside edgeville bank. It's this one: