It means that the cost has been officially and properly noted in the report. For example, if you have licensing fees for Microsoft software used on your company's computers, and you report those fees as a business cost on your accounts, then those fees/costs "have been accounted for." Outside of business, we often use this phrase when taking attendance. If there are 22 children attending an event, and the adults supervising them count 22 children in front of them, they might say, "All the children are accounted for."

There are several samples I may offer you here: He could not account for his foolish mistake. (explain) Who will have to account for the misprints in the dictionary? (take responsibility) Our anti-aircraft guns accounted for five enemy bombers. (consist of, comprise)