Will using a Monobehaviour script versus a non-Monobehaviour script use less memory?

If I had an Item class that did not extend Monobehviour, how much more efficient would it be on space since it doesn’t have to store Transforms, Gameobjects, Renderers, etc?

Only extend MonoBehaviour if your script needs to attach to a GameObject. Other wise regular classes are fine.

The performance difference will hardly be worth worrying about.

UA ate my answer again.

Use a MonoBehaviour on everything that needs to be attached to a GameObject. Use regular classes for everything else.

The performance difference is barely detectable, and typically not worth worrying about.