> I've been thinking about this problem a lot, since I'm trying to figure out how to sustain our open source project.

It’s worrying, and maybe telling, that you don’t mention whom the project benefits. Who are the people who want this project to exist? Those are the people who would be willing to pay to ensure its continued existence and maintenance. Exactly what form that takes is mostly psychological, and will therefore depend on what types of other expenses those people are used to paying, and how they usually pay them.