I don't know, I've heard him come up so many times now but it never really checks out. For one I sort of put him in the category of Jesus Christ... yes he existed, was a wise person and had a great following, but he didn't walk on water or return from the dead despite reported eyewitness accounts that eventually led to biblical canon. Only in this case, Diocles is even older, less known and less written about. Can we really take his income at face value, or is it part of the legendary narrative and exaggerated like happens so often in history?

Hand-wavey inflation statistics over the course of 2000 years of different economic systems and currencies on the basis of a few datapoints just don't work anyway, even if the 35m figure is correct. They may give a starting point, but you have to show a source, a decent methodology to back it up. This is it for Struck's claim (on which he stands completely alone as the only source ever cited in the hundreds of fluff-pieces about this factoid):

> His total take home amounted to five times the earnings of the highest paid provincial governors over a similar period—enough to provide grain for the entire city of Rome for one year, or to pay all the ordinary soldiers of the Roman Army at the height of its imperial reach for a fifth of a year. By today’s standards that last figure, assuming the apt comparison is what it takes to pay the wages of the American armed forces for the same period, would cash out to about $15 billion.

By his same token, take the earnings of the highest paid provincial governor (Jerry Brown at 200k), multiply it by five and you get $1m a year. So perhaps in 15 thousand years he'll get to $15b in earnings.

Or take the grain for the city of Rome for one year. An average person used about 18 bushels of grain per year, currently costs $5 a bushel, so the entire city could be fed with $90m, equal to his lifetime earnings. Not quite $15b either.

Besides, there are plenty of other decent price comparisons. A loaf of bread was 1/2 sesterce, translate it to a $1 loaf of bread and his lifetime earnings were around $70m.

There's also plenty of references to the amount of gold the entire military cost at the time (particularly because many coins at the time were minted in gold, so sesterces can easily be translated to a weight in gold). Gold being a pretty good store of value, it can be useful across for valuation comparisons over time. It would put his lifetime incomes around $100m, too.

And then his comparison, it's the worst and least applicable. He takes his salary relative to the costs of running a certain army at that time. Then he translates that to the costs of the US army at this time. That's not how you calculate the present value. It only says something about the relative sizes and expenditures on the army between these two times, not on his salary. A more apt comparison would be to take soldiers from a country with a most similar level of economic development today, like Malawi, and look at their income. It's about $500, then multiply it by the amount of soldiers his lifetime earnings were able to provide for. Looking at it this way, it'd be like having $14m today in Malawi (without access to the international market products). You'd be filthy rich, but you'd have to spend it mostly on paying people and agricultural products.

Which brings me to the last point: what is even the point of an across-time comparison like this. As rich as he was, he had no flush toilet, no electricity to read books at night, no airconditioning, air travel, a library of millions of songs, movies and books, not the medical care to live past 43 years old, or the ability to spend his wealth on a trip into space or to Mars. The best he could do with his money is have people prepare his food and bath and see to his sexual needs, not all that much more. The richest people in the year 0 can't compare on many fronts with most middle-class people today, in terms of wealth.