Then you reply with "Well I'm sorry but for that amount of money, I could pay for a hotel room and go away for a night with my husband/lover/whatever. You all sound like a lovely bunch of women but as I only really know the bride-to-be, I'll back out now before any further expenses come up, or we could change so that none of us have to share a bed let alone a room with another person that we've not met before."
@shestakingtheurine - please have a conversation with the B2B here. I'd be fairly sure that she would be mortified that her friends are being asked to pay that amount of money and share a bed with someone that they probably haven't met.
Imagine that there will be at least one 'active' sleeper (someone who tosses and turns a lot in the night), there will be a duvet hogger, a snorer, someone who talks in their sleep, someone who sleep walks, someone who gets night terrors or nightmares.
At the very least, everyone should have their own bed. Best case scenario, everyone gets their own room.
But above all else, talk to your friend, the bride to be here. You may find that she doesn't want to do this and would prefer to do something completely different and this is all about what the MoH wants.