Think Dota 2 is currently in beta, but once it's out of beta it's going to be free to play like Team Fortress 2. So, if you're just vaguely curious about it I'd say wait.
If you're more interested than that, know that Valve is constantly adding gift copies to players' inventories. So if you know someone that plays it regularly ask if they might have a copy or five to give you.
And if you're even more interested, I am completely inept at those games so can't tell you if it's any good. The interface seems pretty. Hope that helps.