sorry man, i think you missed the point. de bruyne is unique, he's got no manufacturer, can't buy two or more of him, and as you say, his price would depend on many factors
the underwear has a manufacturer, you can buy many of it in stores, the price isn't that volatile, it can be replicated, the manufacturer usually assigns an msrp to every product, etc
the two goods are nothing alike. footballer's registration rights are much harder to assess than manufactured goods. there's no objective way to value them. 20-120 is a ridiculous margin, and it's completely arbitrary on both extremes. and most importantly, there's not even a short guideline for these valuations, neither of fifa, uefa and national associations have any similar regulation. yet juve is punished based on a rule nobody ever created, let alone accepted and implemented.
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and more importantly, i also forgot my original point lol
you said that books were obviously not representative. books should mirror reality, which are the contracts between clubs. contracts say pjanic = 60m, barcelona pay the fee, arthur = 72m, juventus pay the fee. books represent these numbers, at least this is what can be found in the financial reports, and auditors approved all statements ¯\_(ツ)_/¯