> In equivalent basic good purchasing power, Diocles' wealth would be between approximately $60 million and $160 million.
This appears to be supported by the table at the bottom of this page [2], which claims that a seaside villa in Naples cost about 3mil sesterces/sestertii. That makes the featured article's claim that 35mil sestertii is $15 billion questionable. Adjusting using seaside villa prices, 35mil sestertii (Diocles' wealth) is more like $7mil.
The $15bil figure appears to come from the "enough to pay the whole army for 1/5 of a year" stat multiplied by the modern-day USA figure [3] (by the way, [3] appears to be the source for much of the featured article).
As far as the "enough to pay the whole army for a few months" stat goes, the most common figure I can find for "annual pay for a legionary" during Diocles' life is 1200 sestertii, or 240 for 1/5 of a year [4]. 36mil/240 is 150,000, which does sound like a plausible (even high) number for the size of the Roman army. On the other hand, [2] claims that 1200 sestertii buys about 100 lbs of pork, so that means a soldier could expect to get paid about 1 Roman pig per year?
Cross-era price comparisons are hard!
[1] https://en.wikipedia.org/wiki/Gaius_Appuleius_Diocles
[2] http://sites.fas.harvard.edu/~lac61/ASSIGNMENTS/SectionOne/R...
[3] https://www.laphamsquarterly.org/roundtable/greatest-all-tim...