The number of strikes minted, is a factor. But, mostly it has gotten to do with how hard it was to play/get them. And how tight the machine is. The Palms put a lot of the convention red cap strikes in the strike machine drum. While others put less convention red cap in the strike machine drum.
Think about it. Alot of red caps, less play for them; less cost per strike. Less red caps, alot of play to get them; more cost per strike. And if the machine is tight ?????? cost.
Hope this helps,
Dan