How to restrict access to sub-site with pro sites??

Hey everyone,

I recently posted a similar topic, but its' focus was on pro sites and membership integration.

Basically I plan to create a sub-site which will act as the blog. Ex. mysite.com/blog
This way I can create a totally different look.

But the issue I'm having is that I want to only offer access to this sub-site (or blog) to paying users.
I plan to offer 4 levels using Pro Sites.
Level 1 = free (no access to blog)
Level 2,3,4 = paid (access to blog)

Any ideas on how I can restrict access to the blog/sub-site for only paying users?

Preferably a way that wouldn't require users to register a second time. Looking for a user friendly way.

Originally, I was thinking of restricting access by user role. I was focusing on the "if user is an admin of any site on network" then allow access. But free users would also be admins, and I don't want them to have access.

I hope I'm being clear.

So again, any ideas on how to make this happen?

thank you!!!!