Net Present Value, NPV, is a detailed calculation of what a project is worth in today's dollars taking into account the timing of the outflow (planning and predevelopment cost, development costs, operating costs, working capital, taxes, etc) of dollars and the inflow (product revenue, grants, etc) of dollars. The flows take into account any anticipated changes that would increase or decrease them (a cyclical business, inflation etc). The flows are then discounted for how far in the future they occur (timing) and for the company's cost of capital (what they would earn if they invested the money in a similarly risk project). You end up with a number that shows you if you would make money in todays dollars (positive) or not (negative). The bigger the number the better but the starting premise is that any positive NPV should be pursued as it will make the investing company money. Experience says otherwise and that company's want and are prepared to pay for high NPV projects

The percentage of in-situ value is a very loose rule of thumb that no buyer will ever use. A low grade high costs project might have a negative NPV and no one will buy it no matter what the insitu value of the minerals it contains. A high grade low cost early exploration project might generate a positve NPV and result in a purchase equal to 10% insitu value by chance. The same high grade low cost project that just went into production would have a much higher NPV (due to lower risk on timing, costs, etc) and would command a higher price which would reflect as a higher % of insitu value. So insitu is at best good for rough estimates of what to exoect if you have enough examples but the $'s are in the details.

.... Been There