Simplify the Web Small and Simple things

My wife has been running a successful Pixel Art site for a few years now (http://www.myimagination.eu), but she would like to establish some sort of paid membership system, which would give access to extra areas of her site. She found that the most successful sites on the net were using forum CMS as their online sales framework.

I had a look at Drupal, phpBB, Joomla and Invision Power Board. The first three are open source, and the third costs US$150, a little on the pricy side of things.