While audit fees have largely leveled off the past few years for accelerated filers, data shows they paid a slightly larger portion of their revenue to their auditors in 2009, mainly because overall revenue declined.

The latest data from Audit Analytics shows public companies subject to the internal control reporting and auditing paid on average $569 in audit fees for every $1 million in revenue they earned in 2009. That's a slight increase from $541 per million and $542 per million in 2008 and 2007 respectively. However, the change in the ratio is driven by overall declines in revenue among companies in 2009, the firm said. Actual dollars paid by public companies to their auditors fell to an average $8.14 million in 2009 from $8.57 million in 2008.

Among non-audit fees, the trend is similar. Accelerated filers paid $145 in non-audit fees to their external auditors for every $1 million in revenue in 2009. That's a slight increase from $139 per million in 2008. The ratio of audit fees to non-audit fees paid to the auditor of the company's financial statements remained virtually unchanged in 2009; 80 percent of the fees paid to the external auditor are tied to the audit of financial statements and internal controls over financial reporting, where it is required, while 20 percent is related to non-audit services.

Audit Analytics cites a number of factors that have driven audit fees in recent years. Compliance with Sarbanes-Oxley Section 404, and especially the 404(b) audit of internal control over financial reporting, has been the most significant driver, according to the firm. Other factors include salary increases in 2006 and 2007 for entry-level and lateral-hire auditors compared with other professions, increased litigation risk for audit firms, increased focus on fraud detection and other audit trouble spots such as valuations, restatements, and increasing focus on auditor independence rules.