There is rumor they might make it free, they really really want people to move from xp, plus if they get those xp users over to 10 then they have that many more people hopefully using and buying from their Windows store.
But they have not said yet if they are going to, we wont know that till they release it. MS has never given an OS upgrade for free, but things are changing and they can make more money if they did because of their store. So it is hard to tell what they will do.
Shane