No additional payment to Microsoft required, once you've paid for the Windows Server OS license. This is similar to using a Windows Server box as a web server. You wouldn't have to pay for each visitor to a web site hosted on your Windows Server box.
By the way, I have excellent results with Windows Server 2008 and FileMaker Server 12 Advanced. Very robust, great performance, and almost no ongoing maintenance issue.
I generally agree with Morgan's information, but there is one caveat. If you want to have FileMaker use Microsoft's Active Directory for authentication, users in Active Directory will have to have appropriate licensing from Microsoft for those users. If you only use FileMaker security for authentication, then this will not matter.
Thank you both (Morgan / taylorsharpe) for your answers, very much appreaciated. I can now go ahead and with my plans to setup an "at home business". i will be taking my FM Certified Developer Exam in December and I now see the importance of the support on this site. thank you again.