It would be hard. Like you said they're usually free, which gives people the expectation they'll be free. Then they come across yours, and they'll be surprised

It would be hard. Like you said they're usually free, which gives people the expectation they'll be free. Then they come across yours, and they'll be surprised to see a price. Unless you're really offering something different I don't see it happen. A donate-button could work though, if your work is good people will often be willing to give a few dollars.