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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.

I recommend also using Double Experience scrolls which can be purchased in the Voting store in Edgeville Bank.

The fastest altar is not the one outside edgeville bank. It's this one: