The inevitable cost is zero security. Most desktop OS are not designed with security in mind and are vulnerable in different ways, but it's safe to say that Windows XP's security is effectively zero at this point.

Personally, I only use Linux and BSDs on my machines, haven't use Windows for years. But I keep a Windows XP VM, I only need to use it for not more than once or twice a month, sometimes it's ancient hardware supporting program, sometimes it's ancient government program. There's no confidential information in the VM.