I guess that in the end, this isn't a family with great relationships and lots of love and care and desire to create that loving caring atmosphere at Christmas, where it's about people spending time together and the exact meal doesn't matter so much.
OP clearly wasn't pleased with the arrangement suggested in advance about splitting the cost, although she agreed. It sounds like she arrived determined to be dissatisfied and to look out for things that were disappointing about the whole experience, and was just waiting to be cross about the 'bill' as she called it.
A Christmas meal can indeed be frugal and perhaps it might include canned soup.....and still be a happy, joyous family time. Lots of people can't or don't spend much or have much and have a great time with their families and friends. Likewise, lots who spend loads and loads (regardless of whether they are asking for financial contributions or footing a huge bill themselves) have a miserable time. A lot of it depends on your attitude towards the people - and some will feel annoyed about their families before they arrive and regardless of what happens on Christmas itself, but this manifests itself as annoyance over a particular issue, often money. Others are just so pleased to be with each other that they don't mind that they are asked for cash or even asked for large amounts or even more than was spent, because they don't see it all as about the money and 'getting your money's worth' or 'profiteering' or charging people. It's not a thing if one-upmanship or control or power.
Lotsnofnpeople on this thread felt some seemingly righteous indignation on behalf of the Op. Many don't like the idea of being asked to contribute financially, and seem to expect a very lavish Christmas meal. I also think many aren't quite realistic about the costs of hosting and that there is more to it than a turkey and few vegetables, but also are determined that OP shouldn't pay a penny more than was spent, and that if there is any chance that OP has been charged a little more, this is 'profiteering' and greedy. Somehow they think the host should be extremely generous, but it is okay for the Op or guest to be penny pinching and adding everything up and resisting paying what she's been asked for. Perhaps some people just like a bit of conflict rather than harmony.
When people ask me for Christmas, I'm thrilled. I know I can have a year off the big clean, the bed making, the braving the shops or websites, the planning and the time in the kitchen. I will happily eat what I'm given and be glad that someone else has cooked it. I will bring whatever I'm asked to bring or pay whatever I'm asked to pay and I won't be calculating and seeing if I've been ripped off for a fiver. I'll hope to come away having had a lovely time with some great memories and looking forward to next year, when perhaps I will be hosting or we'll be doing it all a bit different, which is fine. I doubt I'll remember the fine details such as what we ate for each meal or if the food was brilliant or very mediocre, but I won't be there to judge. I think the Op was judging from well before Christmas, and this just doesn't bode well for happy family times. I don't really think the issue is about the money or whether an extra £5 or £10 has been charged.