plural of séstertius depending on mílle) was a sum of money, not a coin; the word is inflected regularly as a neuter noun: thus, tria séstertia = $150.00.
When séstertium is combined witha numeral adverb, centéna mília, hundreds of thousands, is to be understood: thus deciéns séstertium (deciéns HS) = deciéns centéna mília séstertium = $50,000. Séstertium in this combination may also be inflected: deciéns séstertii, séstertio, etc.
In the statement of large sums séstertium is often omitted as well as centéna mília: thus sexágiéns (Rosc. Am. 2) signifies, sexágiéns [centéna mília séstertium] = 6,000,000 sesterces = $300,000 (nearly).