At Dortmund, yes. At Liverpool: also yes but I wouldn't call them bargains anymore. Salah and Mane both cost more than £30m if I'm not mistaken, Wijnaldum same price range. Van Dijk and Alisson were very expensive, so was Keita who was a flop.
He's just good at turning very good players into superstars, and those very good players (usually) require an "average" fee instead of fees north of £60m for teenage superstars or already established world class players. Just like with Konate, I wouldn't call €35m or whatever it is a "bargain", it's just that it might look that way if he becomes a genuine superstar yeah.