Tis a bit difficult to get an exact answer on that, because the offerings change with each release. Further, both client and server often have the same software installed, but server has a gui and a remote management interface for that software.
I would start with <http://www.apple.com/server/macosx/>. The meaty technical details are best found by looking at the items on the right side of the page, where specfic technologies are listed.
I did not find server excessively expensive for its market niche, but I usually deploy client machines if all I need is a simple web server or the like. When I want to manage a bunch of systems remotely, then MacOS X server begins to look attractive.