New screenshots released by Cyborgmatt shows the in-game shop interface for DoTA 2 currently in testing. Indicating that DoTA2 will most likely have a Free2Play model.

Looks like Valve might adapt a similar system like League of Legends, allowing player to purchase different cosmetic weapons and armor for each hero. (Possibly lots of silly hats like in Team Fortress 2 for your favorite heroes)

Source: dota2.cyborgmatt.com/