There is an old NBA adage: Buyer beware the leading scorer on a bad team.

That didn’t stop Knicks president Leon Rose from taking a deep plunge — though not to the max — on RJ Barrett after a season in which he averaged 20 points on 40.8 percent shooting as the Knicks struggled to a 37-45 record.

“The Knicks normally overpay for decent players,’’ one coaching source said. “Is RJ ever going to be more than a low efficiency, high usage scorer? Maybe. But maybe not. That’s the point. But they think he’s got a lot of room to get better.’’

While his new contract extension is a less-than-the-maximum extension afforded to studs like Ja Morant, it still will pay him $108 million guaranteed over four years ($120 million if he reaches incentives such as All-Star/All-NBA/All-Defense selections). He stands to make $28 million in 2023-24 after his rookie deal ends after the upcoming season.