How much money yearly would it take to keep up a castle?

Depends a lot on the castle – size is a factor, the military requirements on the castle are a factor, the social and economic requirements on the castle are a factor, the availability of local supplies and labor are a factor.

Regardless, they were never cheap. During the 14th century, for example, maintenance of a baronial castle was around ¼ of one’s income, and could lead to debts of a thousand marks. In the 15th century, for example, one Scottish lord spent 800 pounds a year on maintenance.

At the high end, take something like Edward I redeveloping the Tower of London in the 13th century for over twenty thousand pounds. That works out to around 16,000 dragons.