Make it relative? Make the cost of renewal about 10% of initial 5-year revenue from the title. If you're still selling it a lot and you intend to continue, this should be easy to pay. If not, it's quite expensive.

The costs should be prohibitively high, unless the work is still extremely productive.

The costs should be tuned so pretty much only Disney would pay the 55-60 year extension for Mickey Mouse related works.

I don’t even think they’d pay for Mickey. Mickey doesn’t really make them that much money anyway, that’s all Star Wars, Marvel, and Pixar (and ESPN). They only keep bribing the copyright extensions because it’s super cheap to bribe a few congressmen - and there’s no organized opposition anyway. They wouldn’t pay billions for Mickey though.

Come to think of it, the comic books that all the big movies are based on, though, they’re all so old that those are the ones that Disney and whoever owns DC will be thinking of while they’re fighting to make sure such a scheme never happens.