I wouldn't go and I'd reorganise the tour with a group of your mutual friends.

If she was hoping for a girls night she shouldn't have said in the first case andrew could come. Sorry but to me it sounds like over the course of 3 months, the plans changed and not only did she not inform you, but didn't inform you of the costs. If I did that to my friends I don't imagine they'd come either. And given that the rego is normally heaps less I understand the financial stress it puts you in. It's not like you just forgot.

Courtesy goes both ways.