It can be as long as a piece of string trying to answer that.
How many are you feeding?
How much will you prep yourself, or how much will you "buy in"?
How many sides do you want, 20 million things or just turkey, potatoes and a veg?
What starters - as some cost a bomb and others are dirt cheap?
Do you have standards to uphold - like it must be organic, it must be turkey, or it just has to be a nice dinner (not even necessarily a roast dinner - lots like big curry feasts!)?
And what do your storecupboards look like? Loads of herbs and spices and flour and gravy stuff and frozen breadcrumbs, or bare shelves?
I can't even put a figure on mine, as it varies, I pick up bits all autumn on specials, lots are from my storecupboard and garden, but I also like expensive smoked salmon (a very special treat to the 3 of us - I wouldn't get that if I was serving loads) and 2 bottles of nice wine.
But things you can do now to save on time and energy and cost include:
Make breadcrumbs from heels and any stale bits of bread and freeze these. If you have a big enough batch before freezing, you can always go ahead and make it into stuffing now and freeze that, to save a job on Christmas Eve.
Dry or freeze herbs from the garden (if you have some). Or if you get a pack which is yellow-stickered but still good, buy them, chop them up and freeze them.
Make soup and freeze that for starter.
You can get various veg whenever they are on special (super 6 or whatever offers your supermarket has) and prep and freeze them. There are lots of recipes on various websites for these (places like Good Housekeeping or Jamie Oliver are pretty good starting points).
You can also ask your butcher for some chicken or turkey bones to make stock now, to be able to make delicious gravy. Especially if you know your butcher well or are buying a lot of meat. Jamie Oliver has a make ahead and freezable gravy which lots on here swear by but I've never tried.
HTH a little even if it doesn't come anywhere near answering your questions.
(M&S are launching their book next week or the week after, they told me today).