Of course, the value of the sesterti must itself have fluctuated over the centuries it was in use. And so if I must narrow it down, I am particularly interested in its value during
- the time of Lollia Paulina who was once covered with 40M sesterces' worth of jewels; and
- the time of Cleopatra, who once drank a dissolved pearl worth 10M sesterces,
both of these stories being from Pliny, Book 9, Ch. 58.
Adam Smith writes in The Wealth of Nations:
When we read in Pliny, therefore, that Seius bought a white nightingale, as a present for the Empress Agrippina, at a price of six thousand sestertii, equal to about fifty pounds of our present money; and that Asinius Celer purchased a surmullet at the price of eight thousand sestertii, equal to about sixty-six pounds thirteen shillings and fourpence of our present money
Smith also writes:
Three sestertii, equal to about sixpence sterling,
which would be consistent with the previous quote, since £1 = 240p.
So according to Smith, 6,000 sestertii ≈ £50 "of our present money". The Wealth of Nations was first published in 1776. Using the Bank of England's inflation calculator, £50 in 1776 = £7,519.57 in 2016, or about US\$10,000 at current conversion rates (according to Google).
So 6,000 sestertii ≈ US\$10,000 in 2017 or 1 sesterti ≈ US\$1.67? Thus
- Lollia Paulina's jewels (40M sesterces according to Pliny) would be worth US\$66.7M today? And
- Cleopatra's swallowed pearl (10M sesterces according to Pliny) would be worth US\$16.7M today?
Of course, I have no idea how Adam Smith did his conversion, but presumably some modern day historians/economic historians have since done better.
(This question was originally posted over at History.SE, but unfortunately the folks there don't seem to know anything about economics. So I'm trying here instead.)