What a shame this all is. In the end it's not really about whether the cost is £40 or £20 or £100, but about the relationship between these people. We don't know how the host feels about it all, but we know Op is insulted and it's not great that even before Christmas she felt resentful and afterwards feels so extreme as to call it insulted. It all seems so unnecessary.
Some people manage a very cheap Christmas with a good meal and some treats and snacks on a tight budget. Even then it's often more expensive than those who've not hosted realise, because it's far more than one meal, especially if people stay over night. On a very basic level, people forget you might need more loo rolls, more soap, napkins, crackers, perhaps toiletries..... Others spend hundreds and hundreds.
And what's this calculating it all to see if it equals £40.... £5 for veg and £30 for a turkey? Surely there's no need to do this. Okay, some people are of the view that no-one should ask for money for food over Christmas, but lots also realise that sometimes it's the only way if the hosting falls to those who might have space but can only afford to do so if people chip in, or if they feel it's fair to share the cost. If you're being the guest and that's the deal, suck it up with good grace. And then I'd say, smile and pay what's asked for.
I'm amazed by those who say you'd eat out for less. Round here, a pub Christmas dinner on Christmas Day for an adult is close to £100 per head. Of course it's a daft comparison because restaurants have to pay staff and make a profit.
Who knows if it cost the host £50 per head and she's decided not to charge the full amount, or if it cost £35 and she's rounded up.......and to be honest, who cares.....is it really necessary to know the exact cost or pay the exact cost. As a guest isn't it good to recognise the effort and burden of hosting and be a bit flexible and willing to do what the host asks, rather than asking for an itemised bill or for the return of the wine? Perhaps they are asking for £5 for per head than they spent, well does it really matter? Is it really about the money or about the family relationship - being able to smile and say 'here's our contribution that you asked for and thanks so much for all your effort hosting us and a lovely Christmas'. Christmas will come around again very quickly and these people will still be your family. Do you want to enter next Christmas and the discussions with everyone remembering that you asked for an itemised bill or refused to pay what they asked for or created a bit of an atmosphere.....do t be the ones to be responsible for that. And if this has really upset you and made you feel insulted (and I'd say you'd need a pretty thin skin and to be hyper sensitive to feel as badly as all this) then just determine that next year you'll arrange your Christmas differently. But don't agree to an approach you feel insulting and then go along and feel irritated and judgemental through the the whole event and afterwards - sounds like you were determined to feel offended from well before the event took place, never mind now.
Pay up. Does it really matter if you are £10 out of pocket compared to what you could have bought the food for? You were able to go away and not have to make up the beds for guests, clean the house before and after and be a guest. It really is t about the exact amount spent.