I highly recommend Adam Jacob’s Open Source Business Models "book" (not really a book, you can read it all online). It’s not very long, but I found it really a good way to step back and think about what’s the model, what to keep open and what to sell, ... and if it could add more weight, I discovered it because it was once quoted/recommended by Mitchell Hashimoto, co-founder of Hashicorp (they also took a little while to find their business model but are now doing pretty well) sfosc.org/business-models
Without more details about the project itself, to see how it fit in the larger landscape, it’s difficult to make recommendations. But the key idea is to identify the competitors and the different kind of users, to try to identify who would be willing to pay for something more, (and from their profile, how much, for which kind of features)
What you describe would fit in the "support" business model, which is indeed very common. The price range though can vary a lot, depending on the clients and product and kind of support. (in other words, without more details, it would be very difficult to give you good feedback)
Hope this helps