I've built many Excel models dealing with production metrics and client billing in the 3PL space; most of them built from multiple cross-linked workbooks containing up to 50 tabs. Most involved used one set of workbooks (three total) to gather production and billing information from client operations, then a fourth workbook collated that information, prepared a variety of reports (metrics by client and by manager; billing points, etc.), and generated a final report that was a billing sheet that was FTP'd to an Oracle Financials AR function.