It shouldn't be hard for the miners to guess that margin, given that the market price of gold can be looked up.
Gold is, for all intents and purposes, usable as money, at least between sophisticated parties such as governments. It lacks some of the features of USD, but if Ghana is out of USD, then those features don't matter to them.
Oh, the mining company management will know what the spot price in dollars is and if the spot price offered in cedis doesn't seem fair. But "we've found a cunning new way to exploit our gold reserves" is a better PR line to the general public than "we're fudging the figures a bit in our favoir*
Look at HN's somewhat sophisticated audience heralding it as a power move against the dollar rather than a country scrabbling around for ways to dig itself out of a financial hole caused in part because the dollar holds value much better than their own currency.
Gold is, for all intents and purposes, usable as money, at least between sophisticated parties such as governments. It lacks some of the features of USD, but if Ghana is out of USD, then those features don't matter to them.