Yes that is true. However, as chiliscales said, it would be ridiculously impractical to spend all year breeding, feeding, and watching the females, incubating the eggs, etc. all just to feed the babies to a king snake. That is not comparable to raising rats if only because of the amount of time and effort required to produce only a few bp babies. Especially with bp's being such an extremely low production snake (in terms of egg count).
I don't know if the OP's plan is to do this (i.e. go through all that trouble just to feed them ALL off to king snakes) or to just feed off the sickling individuals. The OP didn't specify their plans as to this regard. I'm just saying if that is the case it would be extremely costly in terms of time and effort spent breeding, incubating etc. and rats put into the breeding females (even if you didn't pay for the bp babies directly).