Pro Sites (new site registration)

I need to do a specific registration workflow and I cant really achieve it using Pro sites, unless I am missing something ..

First, I need people to be able to register themselves for free as a users only, no sites at this point, thats a built-in network setting, no problem, but once the are logged in, I need them to be able to add new sites for $9 each (flat rate), I created just one pro level but it seems like you need a site already created to be able to upgrade it to pro site. How do I achieve that?