It makes sense now, her husband to be thought it was a great idea since the guests would essentially be paying for the hire of the place. Now many have dropped out of this Most Haunted 3 day extravaganza, it's looking totally out of the window. As they could put similar money on the mortgage and go abroad with family cheaper.
Please please don't feel bad, she's the one in the wrong. Your Mum should have your back, the price of the dress, which is gorgeous btw, shouldn't come into it. She'll be apologising as she needs you more. Her husband to be, is likely saying another drop out, just no. How miserable are they going to be knowing every minute of their big day is going to be crazy money.
I posted what our itinerary would have been, what we expected to pay for, so the guests would have had to put out money for 2 lunches maybe, which they could do where ever they pleased. Even in my wildest fairytale wedding moments, I could never justify guests paying because we wanted a certain location.
A couple of family weddings, one abroad, paid for coach, ferry crossing, hotel fees, plus all meals, this was about 6 years ago. Another a distance away all rooms paid for, dinner, bed & breakfast.
I know that things have changed slightly and the brides family don't typically foot the bill. I would suggest it only feasible to pull her dream off, if she had significant backing. Otherwise you are looking at a big chunk of what could be a mortgage payment. Maybe we were boring thinking of practicalities like that, I got the same money as my sisters towards my first wedding, but I was born in an era when that was expected and saved for. My second they insisted on getting the dress, so I fell in love with a dress that fit the castle fairytale, but ended up with a much cheaper affair by 90% maybe if not more.