Memberstack

Memberstack is a Javascript Library that allows you to add user login and payments to websites in an easy and effective way, so you can focus your attention to building your business instead. It features secure and encrypted user authentication, Stripe integration and gated content with a beautiful user interface.

Memberstack is free in trial mode, where you can set everything up and test before you go live. Once you do go live, the cheapest Pro account costs $25, with a 3% transaction fee (as opposed to 0-1% on other plans), with 10,000 total members. Check out Memberstack's help section to discover more about their service.