check out this site..
http://www.opensourcecms.com/
it lists most of the opensource CMS's and has working demos, including admin areas so you can see what you are getting...
prolly wanna look under the "Portals" section 1st
now, most of these are not really geared for paysite type operations..but many of them can be adapted to do what you want with a little work.
also be aware that for the most part you can't just "plug in" a CMS into an existing site..it's basically a "framework" in which you build your site..so dont be suprised if you basically have to rebuild your site to use a CMS..I'm not saying its not possible to adapt certain areas only to use a CMS..but that gets kinda tricky...
good luck!