Webextends

If you need to grant access to your website only to those peaople that had paid 
a fee or have done something you need before they access: this plugin allow you 
to setup an “access URL” that permit you to access all website contents.
If users don’t access through this URL, they’ll be redirected to another page 
(for eg. a payment page) and then redirected back by that page to the “access URL” 
if they pay or do something you want.

PayPerAccess

Use this plugin to filter the access to your website redirecting the non-auth users to a payment page o something similar.

PayPerAccess vs s2Member – Excellent for All Kinds of Memberships, Content Restriction Paywalls & Member Access Subscriptions