150 million sesterces

Salvete commilitones!

I’m a little stymied by the following passage from the “Res gestae divi Augusti” in which he claims to have given (out of his own pocket) 150 million sesterces to the treasury. Or at least that is what the translation (from a calendar with some daily Latin) says. I do not understand how the “million” is being expressed. Here’s the original passage:

Quater pecunia mea iuvi aerarium, ita ut > sestertium milliens et quingentiens > ad eos qui praerant aerario detulerim. Et M. Lepido et L. Arruntio cos. in aerarium militare, quod ex consilio meo constitutum est ex quo praemia darentur militibus qui vicena aut plura stipendia emeruissent, HS milliens et septingentiens ex patrimonio meo detuli.

According to my dictionary “milliens” means “a thousand times”, and “quingentiens” means “500 times”. Put together this ought to mean “1,500 times” instead of “150 million times”. Is it simply understood in view of the context that greater sums are meant? Even then, why exactly by a factor of “100,000”?

And what in general was the way of the Romans of the late republic and early empire to express the concept of “million”.

Valete,

Carolus Raeticus

“centena milia” is omitted as an implied number // omittatur ut numerus conclusus

http://www.hhhh.org/perseant/libellus/aides/allgre/allgre.138.html

http://discourse.textkit.com/t/in-defence-of-shameless-self-promotion/9550/1

Sestertium is the old 2nd declension genitive plural of sestertius, modifying an implied centena milia (literally: “one thousand five hundred times [each one hundred thousand] of sesterces”). This was how large sums of money were regularly reckoned.

Thank you, Adrianus and Imber Ranae!

Equipped with your help concerning the omitted centena milia, I was able to find a detailed covering of this topic:
http://uts.cc.utexas.edu/~silver/Reference/money.html
Surprising what kind of difference a “milliens” instead of “milia” can make.

Yours sincerely,

Carolus Raeticus