Ein gutes Maß für die Größe der Schattenwirtschaft in einem Land ist die Summe an entgangener Mehrwertsteuer, sagt Murphy, da es sich hierbei um eine Steuer auf Umsätze und Verkäufe handele. Deutschland hat 2015 und 2016 rund zehn Prozent weniger Mehrwertsteuer eingenommen, als es ohne Steuerhinterziehung zu erwarten gewesen wäre

Tossing around billion $ figures makes good clickbait, SPIEGEL,
but it's also dishonest:
income from black markets and the shadow economy can never be turned one for one into regular taxable income.

Part of it is simply illegal, and would vanish under increased law enforcement, leaving only costs for prosecution and punishment.

And another, like tradesmen working unreported on construction projects, would become unprofitable under full compliance with tax and social security.

As far as basically legal activities are concerned, the underground economy exist exactly because it offers lower costs and less bureaucracy.

Take that advantage away, at least some of the jobs that previously got done informally will no longer be affordable.

So round those figures down by at least a third, SPIEGEL.