Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
But the cultivated class of merchants and the vigorous order of landlords were far overgrown by the two classes that gave tone to society--the mass of beggars, and the world of quality proper. We have no statistical figures to indicate precisely the relative proportions of poverty and riches for this epoch; yet we may here perhaps again recall the expression which a Roman statesman employed some fifty years before(52)--that the number of families of firmly-established riches among the Roman burgesses did not amount to 2000.
52. Cf. IV. IV. Administration under the Restoration
The burgess-body had since then become different; but clear indications attest that the disproportion between poor and rich had remained at least as great. The increasing impoverishment of the multitude shows itself only too plainly in their crowding to the corn-largesses and to enlistment in the army; the corresponding increase of riches is attested expressly by an author of this generation, when, speaking of the circumstances of the Marian period, he describes an estate of 2,000,000 sesterces (20,000 pounds) as "riches according to the circumstances of that day"; and the statements which we find as to the property of individuals lead to the same conclusion.
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Reference address : http://www.ellopos.net/elpenor/rome/5-11-old-republic-new-monarchy.asp?pg=104