My best shot at it currently is to have a subscription that allows plugins to be installed, and then just evenly splits each user's subscription fee among the plugins that user has active. There are several ways this could be gamed. For example, you could break what would naturally be a single plugin into several that depend on each other in order to get a bigger slice of the income.
Have you seen or thought of any other models that are really good at this, and hard to game?
I messaged @kepano about being able to support paid plugins. Apparently they can't because of Apple App store rules that forbid third party "stores" (and/or require a cut of the payment). IIRC there was a ruling that walked this back a little bit, but only in the US?
One thing I've appreciated about Obsidian is that they have donated to the top plugins as voted on by users each year. It's a small amount (eg. $25), but it is a nice gesture.
Micropayments could also be a nice gesture, but I don't think they could add up to meaningful income for a developer. There are enough developers that are willing to build great open-source plugins for free. IMO the missing tier is having quality (and vetted) plugins that people can work on full-time.