To save my repos "for posterity" on Github without incurring a $100/month bill, I simply create a master repo (say old-projects/) with all my unused repos in it.
On the other hand CodePlane seems to be a great solution for collaborative work on lots of private repos. This can save big bucks.
I love to see projects like this. I hope it gains some traction and forces Github to change their business model to a more manageable per-GB price.