The inflation adjustment from 1980 to today is about three times.
One of the 'mistakes' some make is to use the peak price from 1980 and apply it to today. That is fine as long as you are looking for the peak price again.
The other 'mistake' on almost the opposite end is to use a low price of the time.
Gerbino for instance in the article just posted, he uses an average over centries in time.
That is fine if you are looking for an average price and especially a price hundred years from now.
To use the estimateed average of $360 for 1980 and extrapolate it to today in my opinion is out of place and quite simply wrong. The circumstances of a times swing the price up and down relative to those centuries based averages. A large portion of investors invest withonly one or two years in mind and the vast majority with at most ten to twenty years max.

The more appropriate price for 1980 is what was approx. the average for the year, of between 500 and 600 ( 560 seems to ring a bell with me). And that period of time is also very appropriate for today. Then it was the Vietnam war costs coming home while now it is the weight of debt. On average we could call it one and the same.

Hence I tend to go with Sinclair's price estimate of $1650. A middle of the road type.
The mitigating circumstances now are that today the situation is way more profound then the late 70's era was, hence it could very easily overshoot that 1650 by a country mile.
Sinclair himself has hinted at that possibility recently.