One number per page type
The value per session changes a lot from one type of page to another. A category page can earn several euros per session and a blog post, a few cents. Calculate a value for each page type and multiply it by its clicks: that shows you which parts of your site pay for the project and which only bring traffic.
It is the first step of the methodology I use for SEO projects. I explain all of it, with an example from start to finish, in my methodology for SEO projects.