shared parties

[needtonamechangeforthis]

Have never shared parties before. However, have agreed to share a significant birthday party with another child. Am being purposely evasive on detail incase she is a MNetter. I thought it would cut the costs. However, the other Mum is seeing it as a way of doubling the cost of what she'd normally invest in her DC's party (which is already double what I would spend) I have said to her, I thought the idea was that we'd cut costs and she's saying ah, well, it would be nice to do x, y and z and anyway I've already booked it. We split responsibilities for who booked what for the party (I gave her things where I thought she couldn't go mad) But obviously I got it wrong. Can't change the things already booked, but how do i stop it getting even further out of hand? Other than walking away from the whole arrangement.