Valve just confirmed on the Dota 2 blog, that it will be free to play.
More here: http://blog.dota2.com/2012/06/introducing-the-dota-store/
Today we’re announcing that Dota 2 will be free to play, and contain an in-game store where you’ll be able to buy fancy gear to customize your heroes. From the forum threads we’ve read over the months since The International, it’s pretty clear that this won’t come as much of a surprise to anyone out there. There are a variety of smaller details that we’ve decided to put together a FAQ to help with, but we wanted to address the two most common concerns right away [...]
More here: http://blog.dota2.com/2012/06/introducing-the-dota-store/