Generally speaking you get 90% of the functionality for 10% of the cost. However, in many cases you get more functionality for a lower cost. For example, many of the open source products "grew up" in the Web 2.0 world, so they were made from day one with security and MASSIVE scale as part of their design requirements.
Very few proprietary products were build out of the box to support deployments the size of Google, Yahoo, Facebook, and eBay. All of these deployments are built on open source for many of the reasons I have been talking about in my blog, because open source provides better security, huge scale, all at a much lower deployment cost. If enterprise and web scale is what you need, open source is the way to go. 
Alternately, it is very important to understand the licenses and support agreements and how you are going to use them. There are some examples where the open source licensing and support can be more than a proprietary equivalent. I have found these examples to be rare, but they do exist (for example if you look at the GSA schedule, RedHat on the same server will actually cost you more than Windows). It's important to know the cost of acquisition is zero, but open source is not free in a production environment, because CIOs running mission critical environments need support and indemnification.
Open source enterprise products are ready to support your mission critical applications, in the operating system area there's Solaris, Linux, in the middleware area there's Glassfish, JBoss, in the database area there's MySQL, PostgreSQL and even in the desktop area...which has been lagging behind in open source, but is starting to gain some ground with over 220 Million OpenOffice users. Government organizations can realize significant savings in support costs by moving to open source products.
Bottom line, you are saving money on the licensing cost, support cost, deployment cost, and manpower to deploy it. It's just all goodness from a cost perspective. Lately I have seen numerous government reports estimating many, many billions of dollars that could be saved by moving to open source.
