Start with the part count. A one-piece rotocast body needs one mould with two halves. Add an articulated head and you have two moulds. Add arms that are moulded separately for a different colour, and you are at four. Each additional part carries its own mould, its own trial shot and its own seam to trim.
Undercuts are the second lever. Any feature that cannot be released in the direction the mould opens needs a slide, a split insert or a hand-loaded piece. Slides are the expensive kind: they add machining, add maintenance and slow the cycle down. Designers often add them without knowing, usually around ears, tails and helmet edges.
Surface finish is the third. A polished cavity gives a gloss part straight out of the tool; a matte part needs either a textured cavity or a post-process. Texture is usually the cheaper answer at volume and the more consistent one across batches.
When you read a tooling quote, ask for it broken down by mould, not by figure. Then ask what happens to the price if the run doubles. A quote that does not change shape between 2,000 and 20,000 units is either padded at the low end or missing something.