How about this?
At renewal, term = max( 1, round( 20 x .97 ^ duration in years ))
Cost = round( duration in years x ln( duration in years)) x median individual income
The total assumes that MII will have roughly constant relative purchasing power.
| YEAR | TERM | COST (x MII) | TOTAL | AVG |
| 0 | 20 | 0 | 0 | 0 |
| 20 | 11 | 60 | 60 | 3.00 |
| 31 | 8 | 106 | 166 | 5.35 |
| 39 | 6 | 143 | 309 | 7.92 |
| 45 | 5 | 171 | 480 | 10.67 |
| 50 | 4 | 196 | 676 | 13.52 |
| 54 | 4 | 215 | 891 | 16.50 |
| 58 | 3 | 236 | 1127 | 19.43 |
| 61 | 3 | 251 | 1378 | 22.59 |
| 64 | 3 | 266 | 1644 | 25.69 |
| 67 | 3 | 282 | 1926 | 28.75 |
| 70 | 2 | 297 | 2223 | 31.76 | (biannual renewal)
...
| 84 | 2 | 372 | 4603 | 54.80 |
| 86 | 1 | 383 | 4986 | 57.98 | (annual renewal)
...
| 100 | 1 | 461 | 10928 | 109.28 |
...
| 200 | 1 | 1060 | 86673 | 433.37 |
...
| 1000 | 1 | 6908 |3197517|3197.52 |
...
| 2000 | 1 | 15202|14199586|7099.79 |
Recall that those numbers are not dollars, but the entire income of one person, working all year. Given that such a person spends no more than 0.1% of their income on a given work, multiply by 1000 to estimate the number of paying fans required just to keep the work copyrighted. If you have less than 60000 copies sold in the 20th year, it is not worth extending copyright another 11. If you have fewer than 106000 sales in year 31, you probably can't afford to go another 8.
In theory, this would allow a work to remain copyrighted for millennia, but at that point, you are essentially employing tens of thousands of middle class workers to do nothing for you at all. The annual renewal bill for a 2000-year-old copyright would be about $500 million now. The Christian Bible, all editions--the bestselling book--has annual sales around $500 million.
But in reality, a one year consumer boycott on any copyrighted work older than 85 years old would probably be enough to force it into the public domain.