The Department of Labor allows an exclusion from the annual audit of assets of the plan which are held by a bank or similar institution or insurance company that is regulated and subject to periodic examination by a state or federal agency. This applies if the institution holding the assets certifies the required information and the plan administrator exercises this option. The exercising of this option allows for a limited scope audit, which is substantially less in scope and costs significantly less than a full scope audit.
Your auditor should show you the courtesy of determining whether the audit is to be full or limited scope before quoting you a price. An auditor can still perform a full scope audit when a certification exists, but the more client friendly approach is to indicate that a limited scope audit can be performed.
However, limited scope audits cannot be used in all situations in which a certification exists. For example, if the certification does not cover certain plan assets, then those assets are subject to full audit procedures. Also, plans that are required to file a Form 11-k should have a full scope audit. The reason for this is that the Securities and Exchange Commission will not accept a limited scope audit report in connection with a Form 11-K filing.