You can purchase Reader Extensions without the LiveCycle suite, yes.
I'm not sure what you mean when you say a server or a workstation. The software requires Windows 2003 Server, RedHat or Suse Linux, AIX or Solaris. If you're running one of those on a "workstation", then you can use Reader Extensions on a workstation.
Pricing information is only available through a sales person. However, its safe to say that the software will run into the thousands or tens of thousands of dollars. There are other posts in this forum that mention that same information. http://www.adobe.com/products/server/readerextensions/main.html
provide more information about the software.
There is no trial version of the software available. However, if you want to try the software, you can become a member of the Enterprise Developer Program. That will entitle you to download a developer license of LiveCycle software, including Reader Extensions, and it will also provide you with 1 year of unlimited email support, to help you get your implementation up and running. The cost for 1 year of the Enterprise Developer Program is $1495. If you have any questions about the program, you can email firstname.lastname@example.org You can sign up for the program
or get more information about the program