Im going to offer an opinion as a divorced Dad with a daughter and how I would weigh up the situation. I would 100% want my daughter there, it’s the start of a new chapter and I’d want her to feel part of it.I think we can all agree that’s not what the issue is.
The boyfriend situation needs some consideration. How well do I know the boy and most importantly do I feel comfortable with him sharing a hotel room alone with my daughter at my expense.
Thinking back to when I was that age, my GF’s parents at the time didn’t want us going upstairs alone, we did sneak upstairs but only when they were out 😇.
They did let me sleep over after we had been dating a while, but in separate beds. Eventually they let us sleep in the same bed but that was after something like 18 months.
Your ex may have doubts as he hasn’t had chance to get to know your daughters BF well enough to be comfortable with them being alone on his watch. Fathers can be weird like that ☺️
If it was my daughter, and I knew and liked the boy and wanted to show my support for them as a couple I would pay. It would at the very least give me a bit of piece of mind for her safety when travelling.
However, I also think there is an opportunity here as her mum to show support for your daughter and to signal to your ex that you trust the BF by offering to pay something towards it.
Best of luck, hope it all works out 😊