The financial cost should be exponential in nature with no die off. Maybe the first couple would be affordable like 40 years worth.
initial - $ 0
10 year renewal - $ 50
20 year renewal - $ 500
30 year renewal - $ 5,000
40 year renewal - $ 500,000
50 year renewal - $ 5,000,000
60 year renewal - $50,000,000
n year renewal - a_{n-1} * 10
Note all of those numbers should be inflation adjusted as needed. Especially considering that most money is made in the first couple of years.
Maybe treat characters and franchises as trademarks instead too. Make it a use it or lose it after the copyright expires.