United made Maguire the most expensive defender in history after reportedly paying Leicester City a world-record £80m (€86m) for the England international centre-back.
The Red Devils had been linked to Napoli's Koulibaly at the start of the transfer window, while De Laurentiis revealed in January that the Serie A club had rejected a €105m offer from an unnamed team.
Discussing the transfer market and Maguire's move to Old Trafford, De Laurentiis told ESPN: "The problem of the cost of players is from England because they gross the most amount of money, more than Spain, Italy, France and Germany.
"If a club grosses £800m, he has no problem to offer £80-90m for one player. There is not a real and fair competition between England and other countries."
"Koulibaly has a release clause of €150m," De Laurentiis added.
"In England, you know, they can spend €93m on a player that I would pay a maximum of €25-30m for. So, if Maguire costs €93m, then Koulibaly is worth €250m?"
Lozano is reportedly close to joining Napoli, who are still trying to negotiate a deal with Madrid for James as LaLiga side Atletico Madrid circle.
"We've been following Lozano since last January," De Laurentiis said. "We know very well that he can be a good buy for Napoli. We've also been following James and others, like the Brazilian Everton [Soares].
"What I like about Lozano is that he's still young, unlike James who has more experience and has played under [head coach Carlo] Ancelotti twice.
"For that reason, he offers guarantees. We must consider who can help the team the most."