Though it seems hard to implement - it is easy. So, today - few tips to decrease your audit costs. Do not expect dramatic decrease, but take into account, that even small decrease in dramatic audit bill could be a serious win. External auditors, please do not read as it would frustrate you 
1) Get the audit plan. There is always something, that supports agreed audit fee. Usually it is addendum to the contract, which lists general activities. Have it on your hands and it would necessary give you a hint on where to save.