RE:RE:God I hope they don’t lose this window as well. You can use as many possible commodity prices as needs be in a PEA. It is called sensitivity analysis. It provides a range of metal prices since there is no one static price that will be locked in stone for the next 100 years.
The general rule of thumb under our current fiat currency system is to take prevailing prices and add about 3% per annum to estimate next year's prices.
Of course, rampant manipulation in all markets screws legitimate price discovery up so no one can tell for certain what prices will be. But, most of us believe prices will only continue to trend upwards.