SP Digital Subscriptions is free. You can use it without any limitation to as many sites as you wish. This is the very same extension we use on this site to sell our products. The main functionality is full, and you can use it to implement a simple paid membership/subscriptions process on any Joomla! site.
The only limitation is the use of invoices and coupons. If you need this extra functionality, you need the PRO version at a very cheap price. Note that most of the users do not need those extra features.