There is a Runtime License for SQL Server: SQL Server 2005 for Microsoft Dynamics. 
This Runtime License is cheaper than a full license. 
The restrictions of this runtime license is that you can only install (one or more?) dynamics database(s) on the SQL Server   (At least ... This SHOULD be the limitations ... I didn't test this)

The Runtime version of SQL Server comes in two versions
   SQL Server 2005 Standard Edition (x86) Runtime for Microsoft Dynamics NAV
   SQL Server 2005 Enterprise Edition (x86) Runtime for Microsoft Dynamics NAV
It does not look there is going to be a 64 bit version, but they're looking into that because they definitally see  the advantage... .

Last but certainly not least: you can NOT upgrade from a Runtime License to a normal license.  This involves buying a full version of SQL Server!

Link about it:
http://dynamicsuser.net/blogs/waldo/archive/2007/10/03/dynamics-nav-sql-server-runtime-license.aspx