What is mold cavity doing to injection mould tooling price?

It is pretty intuitive when you think about it.  Fewer cavities in a plastic injection mold require less tooling work, time and ultimately less cost.  

A reputable, experienced molder will be able to maximize cavitation in the plastic injection mold to maintain the highest level of productivity. 

 In general, most molders recommend creating one mold per part versus creating a family mold.   Family molds are created with various cavities for assorted parts.  They tend to produce inferior products and have more downtime due to maintenance issues.