My own stuff was rather exclusively Microsoft until about five years ago. When I needed to make an interactive website, ASP and ASP.NET just were not going to suit. Too much work for to little value.
So, I looked to open source. MySQL, Ruby on Rails, Netbeans, and more. The one thing that kept annoying me was the false mantra of 'it's open source so it's free and it's better.' Well, MySQL is no better than MS-SQL to me, in fact a little worse and scary in some ways. I use it because it's what I can afford at the moment for a web site.
Rails is night and day better than the pre-MVC ASP.net. Now with ASP.NET MVC, I could see going to it.
You're right about the bottom line. It's the cost of development and implimentation. That includes tools (that are sometimes very expensive), support (that J.Ja pointed out is often more than non-Open Source licenses), and training (that is sometimes just not available).
Keep Up with TechRepublic