In most cases, customers want custom-made solutions. So offering Open Source CMS as a full website out of the box will not do. There has to be a degree of customisation. And this is not a few clicks as someone suggested in an earlier comment. You have to be highly skilled to be able to customise and theme, say Drupal or MODX, with all the fancy plug-ins thrown in. Those slagging off CMS-based solutions, I say go easy on your attacks, for this is a real marketable skills we are talking about.
As for how much you charge, Shadowbox said it well. You charge what you think you are worth. If you can put together a website, CMS-based or otherwise, in 30 minutes you don't bill for 30 minutes. Your price should be what you think is right for your skill set.