I'm looking for a site comparable to Adbux/Clixmedia/Tarbux. I want it to be a similar to ClixMedia in terms of functionality. These are not complex sites.

It needs to be flexible enough so I can change payment and advertisment structure to suit my needs. I need a banner/ad rotation system in place.

There should be enough security to prevent hackers from abusing the system.

Last but not least it should be light on bandwith use and be able to function under high loads.

Preferable in PHP or Ruby on Rails with a MySQL backend. I have knowledge of PHP and SQL so please write clean code so I can modify it to my needs if necessary.

Use of crossbrowser/crossplatform CSS for the layout of the site (no tables). Look and feel like http://www.designevo.com or http://www.linkbucks.com, you know a web 2.0 feel.

Design can be done afterwards by a designer if the CSS structure is in place.

A demo to ensure it's implemented as required before payment will be required.