I know the Membership Pro doesn't support custom upload quota's per user, but I found a plugin that allows you to set a max quota per file and total storage space for certain user roles and custom capabilities:
So now I'm wondering if it's possible to combine this plugin with Membership Pro so that I can set different memberships with different upload quota's. So that if users post more, they need to pay more. At the moment I have a site with a memberships that allows users to be authors and publish unlimited posts. But I would like to ad a max upload storage of media per user(author).