You're paying a huge premium for G-SYNC; it's as simple as that. I doubt you'll find any 144Hz 27" 1440p monitor with G-SYNC for much cheaper than the PG279Q.
There are plenty of 144Hz 27" 1440p monitors that have AMD FreeSync and cost $300-400 less than the PG279Q.
You're paying a huge premium for G-SYNC; it's as simple as that. I doubt you'll find any 144Hz 27" 1440p monitor with G-SYNC for much cheaper than the PG279Q.
There are plenty of 144Hz 27" 1440p monitors that have AMD FreeSync and cost $300-400 less than the PG279Q.