No, I wouldn't (think oh she has decided not to go, I'll pay extra). I'd think she decided not to go why the hell should I have to pay extra!
You committed to going, you have the responsibility of either covering the costs you agreed to or finding or helping them to find someone to take your place.
Suppose half of them decided not to go and just expected the rest to pay double and felt that it really wasn't their problem?
When you make a commitment, then you honour it. If you just don't fancy going ahead, or you leave it until the last minute to book time off and then find you can't, then you make sure the other people who otherwise would be affected - aren't.
I would have agreed with you that you were not unreasonable if you had stuck to what you originally said which was leave me to it, I'll book my own stuff and they had then done it and demanded money and at that point you had said no. But you gave them the money. You could have said no, I told you I would make my own booking. You are going to have to cover that yourself. By paying it - you agreed it.
However, I also think they should help out and try to find someone else, because that's the nice and right thing to do and what a friend should want to do.
Have you begun to ask around to see who might fancy this holiday? Have they?