I was quite curious about the cost of an audit like that
I don't know if you guys are too
However, that information is confidential
The best I could do was find an estimate by searching online on AIs:
From $15 million to $35 million

I had no idea it would be that high
Obviously, I don't have any sources
I presumed it must be that expensive.
If we compare Tether's audit with the other top 5 stablecoin issuer, Circle, the estimate is fair and reasonable. Circle had paid $4 million to $6.5 million to Deloitte & Touche LLP for their annual audit.
Considering that Tether has more than double the assets of Circle, and both Deloitte and KPMG belong to the Big Four, $15 million to $35 million must be more or less accurate.
Not to mention that KPMG's audit must be a lot more tedious. Deloitte auditors didn't have to perform vault audits on USDC or Circle, for example. Whereas, the KPMG auditors even had to fly to Switzerland to manually count more than 146 metric tons of gold bar by bar.