In reading your previous related posts, I was going to chime in with exactly what Jay Fienberg has suggested. I immediately think of how browsers handle RSS subscribing to how a Payment Transaction could be made.
This assumes that the page represents a single purchasable object.
Back in 2006, their was a proposal for rel=payment for more granular usage.
It’s also worth noting that crowdfunding is appropriate here. Meaning, your web app would be free and then you provide new features or game levels based on crowdfunding campaigns. if what you gave away for free is good stuff and garners a good following of fans/supporters, then a good percentage may be willing to contribute funds in order to get new stuff released. It’s the same common tactic used within app stores etc. So it should not be ignored for the open web approach.
Another thing to consider is media packaging technology that can control versioned dynamic network content. As you know, i’ve been paying attention to MAF (http://maf.mozdev.org) as a solution to test.
What this may lead to is having browsers be able to have a web app manager as part of the addon/plugin framework. So you can “install” or “register” a web app into the browser and have versioning etc. So the Browser becomes an App Store. The public website is open and free but the installed/registered version has more exclusive access to content/features…. effectively the pro version of the website.