The cost of an audit is different in each situation and depending on the duration of the audit, the number of workers interviewed, etc. In general, amfori BSCI determines the minimum length of an amfori BSCI audit, but every auditing company defines its own service rate. A complete audit requires good preparation, expertise, and professionalism (prior to, during, and after the audit). 

The scheme manager will communicate to the RSP holder any reasoning that could justify increasing number of auditors, extension of travelling time or additional off-site interviews.

This question will be best answered by directly contacting the auditing companies. You can find their contact details here.