Oli here, from the team developing Memex. Memex is open-source, so the browser extension will always stay free to use.
What we will charge for are some of the services that require us to host stuff. Like backups, multi-device syncing, API calls etc. We will run it as a completely modular pricing model, where you can upgrade on only those features you need. We don't like those usual 3 tier model, where you have to upgrade to the 'monster mega plan' in order to just get one feature :) But you can also completely self-host that, as we will make the server software open-source as well.
Hope Memex can be useful to you. We are running a crowdfund to support its development, where we offer some good discount on the future features in return: worldbrain.io/pricing
Let me know if I can be of more help.
This extension looks like it could finally make it convenient enough to be more commonly useful, and privacy focused enough that I'm willing to try it. Great work! Are you planning to charge something for this in the future, or for extra features? I would definitely be willing to pay for it.