The Dell G3223Q is an excellent option. The G3223Q is a 32-inch, 4K monitor with a maximum refresh rate of 144Hz and support for AMD FreeSync Premium Pro (Nvidia G-Sync also works, unofficially).
Today, you can find options well below $400 and still enjoy refresh rates up to 144Hz with support for gaming features like Nvidia G-Sync and HDR support. While all decent 4K gaming monitors ...
Gigabyte 32" M32UC 4K UHD ($530 at Amazon): This monitor from Gigabyte is adjustable and affordable. It supports a 144Hz refresh rate and 4K resolution output. Got a nice curve to it, too.
At the same time, panels like the BenQ Mobiuz E3210U may end up dropping under $600 again, providing a premium 144Hz punch for much less than new arrivals. The least you'll pay for a 4K monitor ...