The amount needed for airfare comp varies greatly due to uncertainty in the economy, gambling futures and airfare itself. For a definitive answer to your question you should call the casinos and and ask for casino marketing. They will give you a good approximation. Get a name when you get your info and ask for him/her when you get to Las Vegas.
The way things are going now I think you will be pleasantly surprised by the amount necessary. They really want you there!

Good Luck!