Feature Request: access control on the lesson or module level of courses

Would be great to have that level of control - especially useful for selling addons:

Do you also want to buy the Extra Lesson X?

(In my case: I sell the video training, and as a addon a cheat sheet)

Now I need to create a new course for make this work (even if the addon is not worth a new "course"). However, for 1 course only that's ok but in my case I have 20.

But I don't know how difficult that would be to implement that...