Jay-Z -- who reportedly picked himself to headline the Super Bowl -- saw his resale tickets for Friday night's packed Los Angeles show drop by more than two-thirds in the last few days to just $20 ... new data shows.

The resale prices for last night's show at SoFi Stadium dipped by 67% in the 3 days leading up to the performance .... according to stats by TicketData.

The data says that while the price averaged around $61 Tuesday ... by Friday, the get-in price was just $20 -- including fees.

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Even more eyebrow-raising is that the get-in price was down 76% over the last 30 days ... and 93% below the all-time highest get-in price of $284.

Of course ... it's pretty standard for resale tickets to take a dip in price before the show.

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And while one X user posted a photo of a thinned-out crowd at the stadium around 9:20 PM ... another user shared several videos of the arena packed to the gills -- looks like it all worked out for the rapper!

We reported earlier ... the rap mogul -- who is in charge of deciding next year's Super Bowl headliner -- has reportedly chosen himself.