Courtesy of our friends at Audit Analytics, who have just issued a new study evaluating the trends of audit fees and non-audit fees earned from accelerated filers over the past 11 years.
As you can see, it appears that audit firms have found a comfort zone in its split between audit/non-audit fees.
And in this table, "Total Revenue" indicates the top line for the population of accelerated filers in the study and as you can see, fees as a % of those revenues have shrunk nearly every year.
To expand on footnote 3, non-audit fees consist of audit related fees, benefit plan, financial information systems design, tax, and other/miscellaneous related fees.
Compliance Week has a nice summary of the report. Discuss as you see fit.