most of the snakes you are trying to get up to weight will eat easily twice a week. especially the females. once up to weight and they drop their eggs to get them back up to weight you will want to feed them heavy again. Assuming only 1 meal a week per animal I don’t think is good planning. I say plan for the worse (in this case highest cost) and hope for the best. But in this case the females eating a ton is also the best case.
BELs will be a dime a dozen and the potions may not be the as common but still by the point you will be producing they will be far more common than you think. here is a link that outlines what i am getting at....
http://market.kingsnake.com/detail.php?cat=32&de=943718
I could be wrong but i could be right. food for thought.