People who hold copyrights for profit tend create corporations to hold the copyrights, then just pass on shares in the company to their heirs.

These shares are valued the same as any other shares for an opaque, non-public company, with a single owner, whose assets have very ambiguous and widely disparate values, and which has the ability to cease operations for long periods of time and still remain profitable.

In other words, they’re worth whatever the owner wants them to be worth.

This is just my cynical interpretation. But if you think valuing real estate is hard, try valuing copyrights.