Do you have to pay to get Minecraft 1.6.4?

If you own the game already then you do not need to pay for it, If it is on PC all you have to do is launch it and log into your account and it should download the update for you. If it is on consoles the update should of pop up when you start the game.

No it is free. Assuming you are on minecraft 1.7.2, when you open the launcher, create a new profile then from there you can use the old versions