I think that sounds about right. We have 4 adults 1 child and for Christmas eve, Christmas day and Boxing day I do; (same menu every year as PILs don;t like change)
Eve.
Home baked ham, cheeses and hot rolls (breakfst every day) with champagne
Lasagne, fruit and cheese board with white and red wine
Light soup and crusty bread.
Christmas day
As above for breakfast
smoked salmon, brown bread and salmon roe
christmas turkey
cheese board and fruit
christmas pudding
nothing dinner
Boxing day
bacon rolls and sloe gin
leftover cold meats, cauli cheese and peas
a proper cooked pudding
Dinner - usually a soup and rolls.
Plus wine at every bleeding meal.
Easily £300 and with half the adults of the OP.
Turkey - £60.00
Gammon (lots) - £60.00
Cheeses - £(from Sainbos) £40.00
Veg - £30.00
Extras - not sure
Salmon- £15.00
Alcohol- god knows.
It's exhausting and expensive. No financial contributions incidentally from PIL, but they very often give too-generous birthday presents so would more than compensate.