Ozil completed his sensational £42.5 million switch from Real Madrid just hours before the transfer window slammed shut, making him the most expensive German ever.
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"The transfer of Ozil is incredible," Klopp told Bild. "That doesn't happen often, €50m (£42.5m) on the last day.
"Mesut is a grenade. Nobody knows why he was sold, maybe Real needed money.
"But it's insane what happens in England. 80 per cent of the players, for whom the clubs spent €700m (£600m), I don't even know."
Klopp also described Gareth Bale's world-record transfer from Tottenham to Real Madrid as "crazy", but admits if a club want to sign a player the price is irrelevant.
"It doesn't matter how much a player costs," he added. "The question is if you really want him and finally if you can pay. The amounts are crazy, but they are staying in the circle of football.
"Perez wanted Bale for any price and now he has paid €100m (£86m) - maybe sometimes he doesn't even know in the mornings what he will do with his money in the evening."