I paid attention to the new Single Audit (also known as an A-133 audit) initiative by OMB when it was first introduced. Here’s a resource page from the Council of Nonprofits on the basics: https://www.councilofnonprofits.org/nonprofit-audit-guide/federal-law-audit-requirements
One key issue that played out in my community is that a nonprofit subject to a required single audit must have, as a component of the audit, an auditor’s evaluation of whether each program under the audit is being run according to federal guidelines. NOTE: this is about program elements, not finance.
Example: A local nonprofit in my home community had a successful financial audit but failed the program evaluation, resulting in an investigation and determination by the Office of the Inspector General (OIG) of the US Department of Justice. The OIG report required the nonprofit to make programmatic changes in order for the program to come back into programmatic compliance with specific grant and general federal requirements. The OIG investigation, I believe, was triggered by the nonprofit’s auditors noting exceptions to programmatic practice. There was never an allegation of misappropriation, embezzlement, or similar deficiencies in the expenditure or accounting of federal funds.
As for Kathi’s question, I would assume that a single audit would be more expensive that a financial audit, if only because reporting on a review of federal program elements is added to the auditor’s work. In the example I cited, the nonprofit reported total accounting expenses of $8,036 on a budget of about $838,000. There’s no way to know what is and is not included in that figure.
Audit costs vary based on a variety of factors, including the extent to which the client nonprofit participates in filling out the non-financial portions of the 990 (assuming that filling out the 990 is part of the audit engagement). The client can, to an extent, negotiate the extent of their work effort in the audit process, thereby influencing the fees charged.