"open beta" is a bit of a misnomer - if you didn't get into the selective invite phase you had to buy a beta pass for $30. Later on (I'm not sure if dota 2 was already the most popular valve game at this point) people with accounts were given a large number of invites to give away. Now that it's been released, dota 2 is free.
Valve (or whoever is responsible for the beta keys) was giving out a huge number of keys to some gamers. One of my close friends received 32 (!) beta keys, if I recall correctly, a few months ago. He gave one to everyone in our circle of friends.
Not only that, but I told everyone who was interested (myself included) to apply for an invite, and we all got one a few days later, so it was definitely rather open to play.
The price of invites on the Steam Marketplace has been about $0.03 since January (and there were several bots that collected invitations and handed them out to applicants).
