Barclays Center operators/promoters don't make a big deal about discounting, but often they do so to fill the house. So they've sold more than 1,400 half-price tickets to the June 17 Barry Manilow show via Groupon. See this offer.
Groupon typically takes a 50% cut, so the arena is likely taking in about 25% of face value. But that's surely better than giving the tickets away. After all, ticketholders do usually spend money on marked-up food and beverage service.
Note that the Barclays Center did not estimate expected attendance in the event calendar (below) previously released. Though capacity, depending on the stage arrangement, is surely at least 13,000 and likely much higher, they are only expecting 11,000, in the calendar released in June.