One version would be the current offering as it is and the other should have more of less the same features but could be ad-supported. Since WHS indexes all your stuff maybe it could display an ad bar along with sidebar.
Sadly I don't think Microsoft will ever go this route. I don't know any of its products being ad-supported, but perhaps if they move all the online applications (if this is indeed the future of software), we may see this in the future. Who knows :)
WHS should have atleast two versions and one of them should be "free"
