Are you sure you aren't underestimating the cost of what was provided? My shopping bill for Christmas day and Boxing day was just over 400 pounds.
Beef - what cut was it? Was it fresh from a farm or butcher's, frozen from Iceland, Aberdeen Angus?
Did you have?:
Crackers
A starter
Beef
Any other meat with it?
Yorkshire pudding
Stuffing
Gravy
Bacon-wrapped chipolatas
Horseradish sauce
Mustard
Cranberry sauce
Roast parsnips
Carrots
Sprouts
Chestnuts
Roast potatoes
Creamed or other potatoes
Goose fat
Cheeses and biscuits for cheese
After dinner mints
Chocolates
Nuts
Dates
Figs
Did other relatives take everything for the dessert including accompaniments like custard, ice cream, Brandy sauce?
What did you actually drink?
Did you stay and have snacks for tea? The kids didn't have chocolates or sweets? No pork pies, quiches, salads, sandwiches at all? Mince pies?
It is surprising how much things add up to. My Christmas puddings were 12 pounds each and I bought 3 of them. When you add in the accompaniments it soon adds up. Meats cost me about 100 pounds.
I must admit that I wouldn't dream of charging my family for hosting but realise others do. If I had agreed to pay 50 pounds for the family, I'd pay that (not 100) and I would have a word with SIL about changing the goalposts.