How much would it have cost to run Downton Abbey in 1920?

I don’t know the answer, but I just read a book called Fall of Giants by Ken Follett where he discusses a Downton Abbey-like estate and the staff and expenses that would have been necessary. That book might help you figure it out. Also, in the group I was in where we discussed it, they said there’s an article in a British magazine called “The Real Downton Abbey.” If you find that article, you may learn more to help answer your question.

This site provides a good look at Victorian finances.

You would have to find a conversion site to find out in todays money.