Free web content that you can buy in other forms (ebook, paper book) or buy extras for (videos, email support, courses) seems like a pretty common model these days.
In the board and card game world, you've got games where you can download a PDF board or PDF cards but if you want a professionally produced version, you can buy it.
In software, you've got the paid support model.
There's plenty of actually valuable things you can sell for minimal additional effort on top of producing the content in the first place. And remember the bar for "actually valuable" is pretty low: SQLite sells licenses for public domain software after all.