Audit firms will be getting a bill from audit regulators for a new annual fee, with Big 4 firms paying $100,000 a piece to support the regulatory regime.

The Public Company Accounting Oversight Board has put audit firms on notice that they can expect an invoice in May based on the number of public company audit clients and personnel they have. Payment will be due July 31.

But companies shouldn’t necessarily expect an increase in their audit fees as a direct result of the new fee paid by audit firms. “The fee represents a very small portion of our revenues and operating expenses,” said Ernst & Young spokesman Charlie Perkins.

Firms with more than 500 clients that issue stock in the capital markets and with more than 10,000 personnel will pay $100,000 annually. Mid-tier firms, those with 200 to 500 public company audit clients and more than 1,000 personnel, will pay $25,000, and the remainder of firms will pay $500 annually.

In its release describing the new fee structure, the PCAOB said it is following the directives of Sarbanes-Oxley Section 102, which tells the board to collect a registration fee and an annual fee from each registered firm sufficient to cover the costs of reviewing applications and annual reports.

Firms already pay an application fee when they apply to become registered with the PCAOB and approved to audit public company financial statements. Those fees will remain unchanged, except for an increase from $250 to $500 for firms that apply to become registered but have no public company clients.

New in 2010, however, audit firms will be required to provide annual reports to the PCAOB. The new annual fee is introduced in tandem with the new reporting requirement. Registered firms are required to file an annual report by June 30 providing details on the firm’s issuer audit clients, audit reports, fees, resources, relationships, and acquisitions, among other matters. They also face other special reporting requirements that can be triggered by specific events, like a change in ownership or a regulatory enforcement matter from someone other than the PCAOB.

The new annual fee is intended to cover the PCAOB’s costs in administering all the new reporting, the board said. That includes costs to develop the new online system for registration and reporting, estimated future costs of processing and reviewing applications and reports, and maintaining the online system after application fees are exhausted.

The PCAOB said the fee may be revised in the future depending on how its actual costs vary from its estimates and whether there are changes in the number of registered firms that would pay the fee.