OS X Lion Server: $49.99 add-on to Lion with unlimited clients. [Update: OS X Server now costs $19.99.]
Windows Server 2008 R2 Standard: $1029 with 5 clients. $1209 with 10 clients. $3919 with 25 clients.
OS X Server’s feature list is so extensive that I can’t possibly cover the details in one blog post. Over the next few months I’ll highlight key features and how they can take your Mac infrastructure to the next level.