Death Toll does a 1 time burst of damage
Boneyard creates a creature that attacks every turn
N damage is worth 1 attack (until the board is full)
Boneyard = 1 attack per trigger for 3
Graveyard = 2 attack per trigger for
6 N damage is worth 1 attack is worth 3
Y damage is worth Y/N attack is worth 3Y/N
3 damage is worth 3/N attack is worth 9/N
This is all estimating for if every death trigger damages your opponent. This is currently not the case but has been suggested and would be easier to balance.
Recommendation:
Make every death trigger damage the opponent of the controller.
Assuming N=3, change cost to 3|2 this cost will probably have to be increased due to Death Tax continuing to work after the 23rd trigger. (I would guess 5|3
)