Ann: Annual Report to Shareholders, page-15

" and only he know its true value, because we all have to guess at the production cost of the inventory."

if we know the amount of gold stockpiled we can all calculate its value for ourselves, or at least its potential value if it were sold at whatever prices are at the time of estimation. So its value can be determined easily.

What we don't know is how much profit is in there, without knowing its full production cost.
However, if the company is not running out of money or having to take on debt, then the production is at least profitable.

I suppose it may be possible to guess fairly closely at how much profit is in the gold should it be sold by looking at the companies balance sheet and P&L statements and making over a few years, ie assuming the figures presented are accurate.