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... anteissent: se annuum sexcenties sestertium rei publicae largiri.

This is part of a three-word phrase meaning 100,000 sesterces; it's partitive gen. pl. from the phrase "centena milia sestertium," with the first two words omitted; the 100,000 is multiplied by 600 ("sexcenties"), so the product is 60 million. "Sestertium," meaning 100,000 sesterces, can also be declined as a neuter singular noun.