Bit of background: I come from a close family (brother and sister and me) and whilst they had children quite young, I was mid-thirties by the time my first came along.
Over the last 28 years (since I was 18) whenever we have all gone out as a family, costs were always split between all the adults in attendance and I've never had an issue with this. So e.g. 8 adults out for dinner with 4 kids and meal is split 8 ways (or 4 ways per couple). This even continued to when their kids were older and having 3 course adult meals...
Now their children are all grown up and mine are young, I had thought the precedent for this was set and tbh we do mostly split the cost of meals between the adults although I always pay a proportion for my kids and they always accept the money 
Anyway we're looking to go away as a family for Xmas. 9 adults will be there and my 3 children. The cost of the accommodation however has been split 12 ways and we are expected to pay 5 lots. So cost of house is £2,250 and £187pp - so £375 per couple but we are expected to pay £937...
AIBU to be slightly annoyed at this? I know 3 days away is not the same as a meal out and I have said I would happily pay a proportion for my 3 kids (like £225 per portion so we'd pay £675 and each couple pays £450) but the difference of £562 seems too much! Obviously there will be food and booze on top of this cost so all told we'll probably be paying £1200-1300 for 3 nights away when I feel we could pay that for a hotel room where we get breakfast included and someone else making our beds...!
I have raised this with my sister and have been made to feel like I'm the one who's out of order but I really don't think I am, so now I feel like not going which will only serve to upset my kids 