These calculations hurt my head...
First we don't know the all in costs of mining an ounce of gold today, let alone over the life of the mine.
Next we don't have any idea what the price of gold will be over the life of a mine... assuming this behemoth has a calculable life span at all. I doubt I'll live to see it mined out.
Last, and most iimportantly, no one is applying a discount rate to their calculation. A dollar to be paid out in 2050 is not worth a dollar right now.
Suffice it to say the shares of Spinco are free (as someone has pointed out). Whatever they're worth, that's still better than a hot stick in the eye. And plenty of companies give you just that.