Wordpress Multisite Membership

Current: Wordpress Multisite using subdirectories/paths already in place and need subscription/membership help. Need: Create and manage memberships at the network’s main site (the primary domain of the network) and provide/restrict access to subsites in the network. Allow me to sell memberships at the main site that then provide certain access to members-only content on a subsite or subsites throughout the entire multisite network. Need all of the membership levels and users to be managed on the main network site. The subsites look to the main network site’s database to mirror the membership levels available and to check a user’s access. I was originally going to use the Paid Memberships Pro plug-in with the Multisite Membership Add-On but the add-on is $300 per year and that’s too high for this. See here for more on the function: [login to view URL] The core plug-in is GPLv2 open software and so it can be built upon the current Paid Memberships Pro free plug-in.