The idea is to create a reference file of existing subscription based sites that exist within your targeted markets.
The more thoroughly you evaluate existing models and successful websites, the easier it will be to create one of your own.
You want to search through existing membership sites and communities in order to determine how their membership site is structured (traditional, coaching, email based, etc), their price structure, affiliate program details (including commissions offered to those who promote them), as well as the specific content that is being offered and overall value on the site itself.
Is their content original and hard to find?