OS X Server Upgrade Path
OS X Server doesn't follow the same pricing structure as the recent operating systems.  While 10.8 (Mountain Lion) and 10.9 (Mavericks) have both been free their respective server apps have still had a cost associated with them.

The only change from Lion Server to Mountain Lion Server and Mavericks Server was a reduction in price from $49.99 to $19.99.

10.10, Yosemite, has been announced and is following the same free pricing scheme as it's two predecessors but I haven't seen pricing for it's Server app yet.  Personally I'd be surprised to see Yosemite Server cost less than $19.99.

With all of that said.  Apple is typically pretty good about honoring recent purchases when a price reduction or new version is released.  For instance, if you purchased Mavericks Server today for $19.99 and then see that it's been dropped to $9.99 or free next week then Apple will often issue a credit for the difference if you inquire.