The guys over at PCPer have disassembled the brand new ASUS ROG Swift PG27UQ, which is a 27-inch, 4K, 144 Hz, G-SYNC HDR monitor that costs $1999. Upon inspection they found that the G-SYNC module inside is a newer one compared to the one that is used in 1st generation G-SYNC displays (which does not support 4K / 144 Hz / HDR). This module is powered by an Altera-made FPGA with the model Arria 10 GX 480.
This part is sold in low quantities for $2000 alone at Digikey and Mouser. We can assume that NVIDIA is buying these in the thousands, so PCPer suggests the price of this chip alone will add $500 to the cost of the monitor. The price is further increased by 3 GB of DDR4 memory on the module. Then you also have to factor in the licensing fees for G-SYNC itself. So this explains why these monitors are so expensive.