Verification is what distinguishes a researched figure from an estimate. The consultant records where each price came from — the dealer, the distributor, the rebuilder, the auction result — and keeps it with the line it supports. Months later, when a number is questioned, the answer is a source rather than a recollection.
This matters most on the categories with the thinnest published pricing: industrial machinery, specialty property and anything with a genuine secondary market rather than a list price.