According to author and SME John Salek in his book Accounts Receivable Management Best Practices, “World-class performance is for the credit and collection function combined to be 1/10 to 2/10 of 1 percent of revenue.”
AR & O2C Network has a five-year-old study that includes Cost of AR as a Percent of Sales for which the median was 0.3 percent – see https://www.thearnetwork.com/best-practices/benchmarksmetrics/results/benchmarks-leading-ar-metrics/.
We found one source of data that cites “cost of the credit department as a percent of sales” – this collections firm offers some figures, though the page does not provide a definition of “credit department.”
You might try AQPC, which appears to do fairly granular benchmarking, though we do not know if they would provide specific data without membership. But we found this resource: APQC Open Standards Benchmarking Measure List.
Further, while not providing that specific metric, the 2014 Benchmarking the Accounting and Finance Function report by the Financial Executives Research Foundation and Robert Half may be of some interest to you.