I don't know an exact number, but having worked in Big 4 audit previously, a typical audit will usually have at least $100k in recurring audit fees. Any smaller than that, and it's probably not worth it as a standalone client.
A few exceptions are probably for :
- international referrals from another office of the firm
- superstar management - I knew an audit partner who wanted to build a relationship with a certain finance executive who had been at several notable companies and wanted to showcase the firm and his abilities. I suppose you could equate this to taking a discount to work with/for someone like Elon Musk? The "marquee client" effect. Also with key relationships comes making a good impression with a PE/VC firm, who then retains the auditor for multiple companies in their portfolio(s) because they now have a relationship.
- future growth opp - notably in tech, but also in other growing companies, where they expect fees to grow substantially (ex. typical of pre-IPO, take it public, and then stay on)
I'm sure there are other situations, but those are the typical ones where the firm would probably accept lower fees in early year(s), but I still believe they don't want clients that are expected to plateau with <$100k in fees annually... at least not for audit. The risk assessment process, and high fixed costs in the operating structure just isn't built to squeeze in small audits. At least, not yet... but maybe one day.
