People have spent more time in Dota than most retail games out there. That one map is more fun than all of valve's products combined except for TF2 and CS. Would I pay more than 99 cents for a better version? Fuck yeah.
Then again, would it have been able to build a large userbase if it had started out as something you'd have to pay for? Doubtful.
Don't get me wrong, I definitely think something like DOTA is worth paying for in the state it is now - in fact I plan on getting HoN - but I doubt it would've been able to establish itself in the first place if it had started out with a price-tag.