OE+FW is 2 cards + 6

+ mono snipe + 5 damage per turn, or 1 card + 8

+ mono snipe + 5 damage per turn.
Grave Lurker is 1 card + 5

+ duo snipe + 3 damage per turn.
There is a discrepancy of 3

. 1 of these comes from the duo cost, and the other 2

probably come from the less damage. The ability to be untargettable by other creatures only is worth around 1/2

if I had to say, so 5

is fine.