Looking for the best plugin to create a paid multisite option

Hello,

I’m looking for a recommendation for the best plugin to create paid membership functionality on a multisite. Ideally I’d like to have the main site be free and then have the plugin protect functionality on a number of subdomains associated with the site. (ex. have an article directory subdomain be a paid section of the site)

I’m curious what recommendations the community would have to enable a paid section to multisite in order to protect registration and specific subdomains.

Thanks for the help,
-Brant