It needs to be free. An online "college" is a joke in the eyes of the world, so you shouldn't even try to compete in that segment by trying to charge money.

You should instead focus on teaching people the subjects. Think of it as a secondary tool that students could use to learn the material on their own. I'd focus mainly on problems, I think thats the only way you actually learn anything but doing examples.

You also should make it competitive, have different rankings and achievements etc that the students can strive for. i.e. Algebra Level I, Level II, etc. Each level would have a different set of questions, that get harder and harder.

Then let users challenge each other i.e glen vs mike - Algebra III. The users would compete to finish the 10 questions, and the winner would be either the person who got the most answers right, or if its a tie, the person with the fastest answers. Give different point values for tests based on how hard they are.

Then help users further, by letting users offer tutoring services. And to compare, the users would be able to see profiles, and see what type of achievements the "tutor" achieved.

But this is mostly just brain storming off the top of my head