Maybe incorporate an Ubuntu-style 'suggested payment' part to the signup? You could have one or two reminders during the course if they choose 'Free'. Or supple

Maybe incorporate an Ubuntu-style 'suggested payment' part to the signup? You could have one or two reminders during the course if they choose 'Free'. Or supplement with paid-for content that goes beyond the course? I agree that it's a lot of work and wouldn't be unfair to seek some compensation.