Using flickr as a example. They offer free and pro. Instead, why not have two web fronts (both using the same backend still). One is called Flickr and would be free, but the other would be Blingr which is paid and looks completely different, a new brand.
A good real world example is Toyota + Lexus (imagine on the backend it's the same people working).
Why do this?
It's easy to implement, and you can really tap in to the folks who have ego and love exclusivity and want to spend. Plus it's like having two separate companies/fronts now.