It can't be worth $60m if dollars no longer exist.

Now, if you want to estimate purchasing power compared to current USD value, you would need to look at total currency supply today divided by the expected supply of Bitcoin that could be used as currency (that is, Bitcoin that isn't lost/burned), and also estimate growth in productivity between now and this hypothetical date.

In other words, it would be not much more than an educated guess. $60m/Bitcoin doesn't sound unrealistic, though.