How is costing determined?

### Describe the feature you'd like

This relates to #4233, specifically [this comment here](https://github.com/input-output-hk/plutus/issues/4233#issuecomment-1021086335). Clearly the `mempty` costing is a default, and probably not accurate for the SECP256k1 builtin required by the issue. However, I'm not sure how costing is determined, or what needs to be done to determine it. Is this documented anywhere? If yes, it'd be good to have it more visible; if not, what is the process of costing?

### Describe alternatives you've considered

Currently, you _can_ define all costs as `mempty`, but this is clearly not accurate, as operations can have very different costs.