A client has been told windows 10 does not work with windows server 2007 and that they need to pay 15,000 AUD (Australia) to upgrade.
They might be right if the client is on a volume licensing program. Some components for the activation of client PCs do not run on all versions of Windows Server. I don't know which versions are supported for the activation of Windows 10, but the minimum version is likely either Windows 2008 or Windows 2008 R2.
Also, some volume license programs have limited downgrade rights. So the company might need to move to the current version (or one or two behind) for compliance rather than technical reasons.