The Server.app for OS X Mavericks has a price tag of $19.99. Some websites mention it to be free for Developers who joined as iOS Developer or Mac Developer.
How do I get the redemption code for the OS X Server.app on Mavericks?
Here are two Articles on the topic:
Apple promoting automated continuous integration to iOS developers, providing free OS X Server copies | 9to5Mac.com
A power user’s guide to OS X Server, Mavericks edition | Ars Technica