Finding Database Default Locations
Most SQL Servers are configured for the non-system databases to exist apart from the system databases.  That is what is normally meant by the default path.

You can lookup some code for finding this at:

http://www.codeproject.com/Articles/684020/Checking-MS-SQL-Server-database-files-default-path

Also, Alex Aza has provided a popular post over at StackOverflow. The key is to get the data from the registry keys where they are stored.  Here is a snippet from Alex's longer script:

declare @DefaultData nvarchar(512)
exec master.dbo.xp_instance_regread N'HKEY_LOCAL_MACHINE',
   N'Software\Microsoft\MSSQLServer\MSSQLServer', 
   N'DefaultData', @DefaultData output

declare @DefaultLog nvarchar(512)
exec master.dbo.xp_instance_regread N'HKEY_LOCAL_MACHINE',
   N'Software\Microsoft\MSSQLServer\MSSQLServer', 
   N'DefaultLog', @DefaultLog output