Callaway argued that the KPMG approach was needed only if it had committed an error in setting up the fund--and that a change in return estimates wasn't an error.
No one, not even the Soviets themselves, knew exactly how much it cost but it was a large enough figure that the best estimates about the Soviet defense burden have a margin of error of around 2% of GDP.
As students, you should keep in mind that all these numbers are estimates based on samples, which are thus subject to sampling error (as well as other types of errors).