Imagine running Grinch's Little Helper only with quantum pillars.
You've got the first turn and you start with one Grinch's Little Helper and 6 quantum pillars.
You play the quantum pillars and end your turn.
We assume that they produce 3
(best case scenario) and your mark 1
.
So you've got 15
and your 4
needed for playing the card.
On your 2nd turn you draw a pillar, you play it and the creature, then end your turn.
You get another 21
and an
from your mark, which makes a total of 37
.
On your 3rd turn (also assuming, that your enemy has no CC or quanta denial), you can use the ability 9 times.
If you've drawn a pillar again, you can use the ability from now on at least 6 times every turn.
That's the denial from 6 devourers starting at the 3rd turn. Even that was imo much to handle for a mono.
With a cost of 4, you can use the ability in the same scenario 12 times at the 3rd turn and on every following turn 8 times.
I know that this scenario is unrealistic and that you shutdown yourself, but it lead me
to the thought, that a ability cost of 4 would be too op against monos in unupped PvP.
I'm still not sure if a cost of 4 or 5 is better, I just wanted to explain why I chose 5 over 4.
If you guys still think, that 5 is too much, then I'll change the cost to 4.