Required by whom?
If your main concern for having an SMS is the fact that several ICAO countries are requiring one, there remains some uncertainty on how they will determine that. Almost all indicate an ISBAO certificate will suffice and that would necessitate an ISBAO audit. Otherwise you may be leaving your crews to debate/negotiate with ramp inspectors while poring over whatever documentation you have on the aircraft as the FAA does not plan to get in the business of certifying part 91 safety management systems.
But considering audits in general... Suppose your company or corporation has solid understanding of bizav ops and management throughout its leadership structure - great! But there are some that don't really have a handle on what the AD is up to down there under Travel, or Facilities, or where ever they have the flight department in their structure. They are the ones that appreciate having a third party with expertise come check things out. And in the first case, it is often simply a matter of protocol or "best practices" to get audited. Now in the bad old days, an auditor would be selected and who knows what kind of criteria he or she used to make their assessment. At least now with ISBAO, you have the playbook up front. The standards are there in black and white and if they get something a little off or even completely wrong, there is a provision where industry feedback through the advisory board will correct it.
So if you are looking to operate your flight department safely and actively manage risk, good on you. If you are able to incorporate what is becoming an internationally recognized SMS structure in the process, even better as you will expand the places you can go. How you go about convincing these other countries you are SMS compliant is up to you. You may find that an ISBAO certificate makes sense but it is merely the culmination of the important effort expended up front to establish a safe operation.
As to the cost of an audit, $1K/day for an experienced professional with specialized skills is a nice round number; 2-4 man days on site with a day either end for prep and report writing plus expenses and it isn't hard to see how you can get to $3-10K depending on size and complexity.
If you are just looking to check the box, it may be a frustrating experience. If you keep your eye on the real objective and have a handle on why things are the way they are, it may go down a little easier.
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