Signup Code for Blogs but not user registration

I need a way to restrict access to registration of new blogs (they would need a signup code or something like that), while allowing anyone to freely sign up to be a user of a site in the network.

Sign up Code almost works, but I need it to only be used for creating new blogs. Is there a way to do this.

If not, I had the thought to use Pro Sites but a) that seems like overkill and b) I don’t want to charge for the creation of a blog, just restrict who can create one. I guess with Pro Sites I could give people that I wanted to create sites a coupon code, making it free, and then charge a ridiculously high fee for anyone else.

Any suggestion.