Selling screenplay rights to a production company. What is the customary industry standard price (average)?

In 2021 all rights to Disney's film "Blackbeard's Ghost" will revert back to me. I have a screenwriter who wishes to enter into an option to create a sequel. I have a proposed contract from the screenwriter's attorneys and I have offered a $1000 option for an indefinite term. I am looking to make money once a production company is interested. I have not signed any contracts yet or received any money for the option. What I want is a set percentage from any monies realized from this project. What would be a fair percent for me to expect? What would be the customary standard price paid to a screenwriter for a screenplay sequel of a very popular Disney film?