The number of days is fixed by ANAB, auditor day rates are pretty much fixed due to availability these days...
Sorry, Andy, but this is not correct. ANAB does not fix the number of audit days. Each CB is responsible for estimating the audit days, according to a combination of the IAF MD5 document and AS9104A, being scheduled for replacement when AS9104-1 comes along. Actually the ICOP oversight stakeholders have identified significant variation in estimation of audit days and the audit day estimation process is one of the focus areas during ICOP Oversight and AB audits. It is a well known fact that some CB's allow for significant reduction of audit time without proper justifications, because of the competitive nature of the certification business.
As for the CB day rates, there is also some variation. Proximity of auditors and associated travel expenses can be significant.