It is assumed that two burners are lit while using the gilded altar and does not include the cost of clean marrentill.
On release, hardened dragon bones were called adamant dragon bones as only adamant dragons dropped them, with the examine text "Dragon bones infused with adamantite ore." They were changed to their current name on 10 April2017 due to gemstone dragons sharing the same bone drop.
A player choosing to use hardened dragon bones all the way from 1-99 Prayer, using a gilded altar with lit burners and no other experience boosts, would, if all the bones were bought at the current medium price on the Grand Exchange, spend 280,886,816.
Additionally the first bone would take them straight to level 5 Prayer.
Using the ectofuntus would cost 245,775,964, but would take far longer to do. Simply burying the bones would cost 983,103,855.