The GameWorks integrations, and that includes HBAO+ and VXGI will probably never become plugins, unless of course the rendering pipeline is opened up more to plugins (thats up to Epic to do), as it requires integration into the actual rendering pipeline, and plugins only have access to a small part of the rendering.
As for the licensing cost, I will leave that to and to answer. But I guess you could go and take a look at the NVIDIA Developer website.