Now, this doesn't include many of the other services one needs. Such as mail, web, database, news, and network management software. Software to provide these services is purchased separately. Other costs, such as administration, vary. The best bet is to get someone who really knows what they are doing.
Some software is better than it's commercial counterparts.
For example, let's look at PC-NFS pro and Samba. Both provide file and printer services between PCs running Windows and Unix machines. PC-NFS takes the approach of bringing Unix protocols and methods to Windows. Where as Samba uses the flexability of Linux to bring Windows to Unix. In my personal experience, I help manage three PC and four Sun labs, the former approach is more difficult to keep properly running.