Vote!
x350 (same as combat XP rate - 500 dragon bones)
x300 (603 dragon bones)
x250 (724 dragon bones)
x200 (905 dragon bones)
x150 (1206 dragon bones) (742 hard mode)
x100 (1810 dragon bones)
x50 (3620 dragon bones) (742 ironman mode)
your rates are so wrong. At 350X exp 1 dragon bone = 88200 (OSRS = 252 exp), so it's 150 bones for 99 not 500.
Basing it off burying the bone, not what gilded altars would give if they were to be added. Skipping gilded altars in favor of basing the rate on burying will make it easier for players to train prayer if they're getting the bones themselves while still allowing them to be sold to other players so they don't have to do the work.
Voted x150. Also, if possible, would love to see the noted dragon bones from slayer tasks again.
There's always prayer resetting (defence resets have already been suggested and training isn't even out yet), and burying a bone even at the x350 if people even want it that high would put you below protection prayers.
EDIT: Didn't expect someone to actually select x50 lol