Restricting specific page templates within ProSites.

Can anyone give me the basic outline of a code I could use within a theme's page template to restrict it from being useable depending on ProSite level? For instance, someone with page creation ability tries to switch the page template they are using to one that only works with PREMIUM, and so they get a message when viewing that page live that they need to upgrade their blog in order for people to see that page. I'm familiar with creating page templates within themes, just need the specific line of code to restrict a template's use specific to a particular ProSite level. Thanks for any help you can provide.