Don't get locked into a proprietary CMS
About 20% of our new customers come to us to have an existing site re-built from a proprietary CMS, into an open source CMS such as Joomla, Drupal or Wordpress; or Magento or OSCommerce for eCommerce sites.
I have this conversation at least once a week with clients. About 20% of our new customers come to us to have an existing site re-built from a proprietary CMS, into an open source CMS such as Joomla, Drupal or Wordpress; or Magento or OSCommerce for eCommerce sites. They feel locked into their CMS, and their provider. We're exclusively an open source house, so I am biased, but strongly believe that there is very little advantage in using a proprietary CMS to build any website.
The amount of work required to build a site in a proprietary or open source CMS is generally the same. It could be argued that a proprietary system is intimately understood by the developer, and as such the site can be built faster; however many development houses have built up just as much knowledge in Open Source systems, and so this isn't a challenge - as long as you choose an agency that specialises in your chosen open source CMS.
When building a site using a proprietary system, the client is usually paying to help cover the costs of development of the software development exercise. This means setup costs are in fact higher, even though the effort required is similar.
Most proprietary CMS systems have licensing costs attached to them to cover the cost of maintaining the software. Also, for any additional components that need to be added to the site, these typically also attract their own maintenance costs. We've recently re-built a website for the same cost as the annual licensing fees the client was paying - and this was a substantial site (tens of thousands of $). What's more, the new site is far better!
Open source code can be edited by anyone; anyone that has the right skills of course. You can take your site to anyone that is skilled in your particular system, or language, and have them make changes for you. Alternatively, if you run a proprietary system, edits can usually only be done by the provider. This of course means less access to appropriate developers, and typically higher prices. Also many proprietary systems will only implement changes that can be rolled out to their entire customer base, so that they don't have to maintain too many different versions of the software; so getting customisations can be difficult.
Intellectual Property (IP) Ownership
Most open systems are covered by the GNU General Public License (http://www.gnu.org/licenses/gpl.html), which basically means you have the rights to do whatever you please with the code, and any customisations that are applied under contract. On the contrary, most proprietary systems are owned solely by the developer.
Mentioned already in flexibility, open source sites are able to be picked up and shifted to just about any other hosting/support provider. Proprietary systems can often only be run on the developers environment, as they like to maintain ownership of the IP.
It has been reported that Joomla alone, which is the most popular of the open source CMS tools, is responsible for 1.5-2 million websites (out of about 16 million active CMS driven sites). Wordpress is probably responsible for even more! So these open source tools are also very, very widely used, being used for some of the most high profile sites on the internet.