> A $7M villa, on the other hand, would be only ~230 years of labor for a US solider paid $30k a year.

Your parent comment says that, adjusted to modern villa prices, Diocles' entire wealth -- 35 million sestercii, enough to buy 12 standard seaside villas -- would amount to $7M. This would put the price of one villa at only $600,000, or 20 years' labor at $30k / year.