I have full physical custody but my ex may be moving out of state and wants me to pay for half of plane tickets if he does. I won't stop him from moving but I don't feel it is fair for me to have to pay half the expense as he wants her to fly to visit quite often. Does anyone know how CA courts decides who pays for this?

The ex made the choice to move. Now if you had moved and there were court ordered visitation, THEN I could see him wanting you to contribute. But since it was his choice to move, then he should foot the bill. Let him take you back to court if he doesn't like it. I doubt he would win.

Generally, since he's the one who chose to leave the state he'd be the one to assume the travel expenses associated with visitation. He might try to get the court to order you to pay half but I doubt that the judge would agree.