We check in on a regular basis to see progress of the new Avid Marketplace, where plug-ins can be purchased and rented and then installed from within Pro Tools. You can see a demonstration of Avid Marketplace here

Currently the Avid Marketplace is showing just Avid plug-ins and no content from third party vendors.

Avid have just released plug-ins for the free version of Pro Tools, Pro Tools First, currently only available on annual subscription.

Checking on the Pro Tools First FAQs from Avid it says this:

 Q. Can I use any AAX plug-ins with Pro Tools | First?
A. Over 20 plug-ins are included with Pro Tools | First including the Xpand!2 Virtual Instrument. Pro Tools | First supports AAX Native 64 and AAX AudioSuite 64 plug-ins purchased from within the in-app Store. Check the description/specs in the Avid Marketplace to confirm if an iLok is required. Plug-in bundles which include ‘First’ in the name are specifically designed for use with Pro Tools | First and do not require an iLok. These plug-ins should download and automatically install, without needing to re-launch Pro Tools. Only plug-ins purchased through the Avid Marketplace with your Avid Master Account will work with Pro Tools | First. All other plug-ins will remain hidden and unavailable at launch. If you purchase plug-ins for Pro Tools | First then upgrade to Pro Tools, those plug-ins will be available as the AAX Native 64 format is exactly the same for Pro Tools | First and Pro Tools.

Based on this we would expect to see versions to purchase at some point rather than just rent.