I don't think it's possible to have objective metrics because the context of the development is crucial.
Let's take 2 examples:

1. A text input is added to a form to add a piece of information

   • It doesn't look like much, but to validate this addition, the developer had:
     • to modify multiple files in the front (preview cards, details page, edit form, ...)
     • to update the database model and may be write migration scripts
     • to update the api because it's a micro service architecture
     • to write tests because the project requires it
     • to update the admin website

2. Add an error logger in the backend

   • This looks complicated, may be not, it depends what is the tech stack, etc. To do this the developer had to:
     • create a little plugin and do a bit java aspect programming. So really fast thanks to the architecture of the project

In case 1, the added information is in fact buried in the website and it's barely used. But it required a ton of work because the project is huge and complicated
In case 2, it's a vital part of the system but it required very little work.

But from an external point of view case 1 seems really important like case 2 not so much if we don't really know or understand the context.

Imho, it should be the project owner decision. He's the only one truly capable of judging the impact of the added feature.