I took the Purdue course, and I liked it. You don't have to be a Purdue student to take it. I think I paid around $800 for it (not including book), and it can easily be completed in 3-4 months. The professor is really easy to contact via email with questions, and I thought the material was appropriate difficulty-wise, but mainly focuses on LA. I have taken biochem, though, which probably made it a little easier, but those who haven't taken biochem could do just as well.