I believe tickets are $45 for Friday and Saturday, and $30 each for the three sessions Monday ($90) total if you ant to go to all 3. They are available at the door or online through USASF. You can also buy the park hopper pass for friends and family which includes entry.
That said, check out the Worlds Spectators thread on the All-star board. There was a lot of overcrowding problems last year, and I believe USASF is planning on clearing the venue between each session. There's a lot of concern on people buying tickets and not being able to get in. There's more information there than I care to try to post here.
Cheer - Worlds 2013 Spectator ? | Fierce Board - The Voice Of Cheer