I agree as well.
A $4000-$5000 quote is really only looking at the sq footage and not taking into account the sheer amount of liability involved IMHO.
I would also image in the main electrical panel is the size of a commercial structure, with 5 to 10 large sub panels.
I’d assume it has 5 to 10 tankless water heaters. And several commercial grade HVAC systems, if not 10 of them.
Every shower, every tub is going to have high end luxury features. Who knows how many tub are jet tubs.
I’d think at minimum an inspector would want a master electrician and an HVAC specialist with them.
And finally, the buyer of such a property isn’t going to care about SOP, they’re going to want everything looked at. No disclaiming the helipad just because it’s beyond the normal scope of a home inspection.
I would have to think a fee in the $20,000 range would be more appropriate.