You definately won't find a programmer willing to do a custom backend for you for $175. That being said, $125 is also fairly low for a design. Now I've never used Mambo or Joomla, but I have skinned PHPNuke and PostNuke, would they be as easy to skin? If so then $300 seems much more reasonable if all you want is a CMS skin (assuming Mambo and Joomla are as easy to skin as other CMS's).