Just ignore the temper tantrum, which is easier said than done.
If she brings it up on the phone, you just calmly say "we are able to pay half right now. Let us know when you have your half and we will book it. Until then, let's table the conversation. How is your dog?" If she brings it back up, you say"mil, you know what we are able to pay. Now let's either change the subject or get of the phone".
If you hear from other people directly you can correct any likes she tells. The more matter of fact you can be ("yes,milis correct, we are only able to pay for half her ticket with all the baby expenses, but we hope she'll save up soon since I know grandbaby would love to meet her") just calm and truthful.
People will choose to buy into her drama or not. And if they think you are horrible for not paying her way completely, then do you really want to be friends with them anyway?
The more you can get your h to handle it,the better. If he can firmly tell her that he made the decision for the benefit of her grandchild,it might help. Probably not,but it might. And if it doesn't, then she is a lost cause.