[rmc]

An easy formula is just based on the age of the work. So you get (say) 20 years for free, but after that you have to pay more. Disney wants Mickey Mouse? They'd have to pay millions per year. If it's not worth it to them, they can just not pay it.

[IanCal]

I'm a fan of this, it seems easier to apply and harder to hide how much you say you're profiting from it. It also represents the trade with the public for protection. The longer it's out of the public domain, the more you have to pay the public.

You could have this increase as a percentage each year, allowing a slow ramp up and much higher fees for something that's been kept for 100 years.