First, note that we cannot compare prices because no other company has a rooftop whole house fan. But we can look at the prices for attic mounted fans.
The two top suppliers of attic mounted whole house fans are Thorwaldson and QuietCool. There are others but no one else has fans that pull over 6000 CFM. You will note that Thorwaldson has the only rooftop mounted whole house fan.
Looking at the attic mounted systems-
The list price for the top of the line QuietCool ES-6400 is $1,849 or 29 cents per CFM.
The list price for the best Thorwaldson ES-6400 is $1,795 or 28 cents per CFM.
Because you have no attic, you avoid the 8 feet of flexible duct that those fans must have.
The result is you get 8000 CFM and at 28 cents per CFM the list price is $2,240.00