The cheapest 4K 144Hz HDMI 2.1 monitor is down to £399 at Currys | Rock Paper Shotgun
Amazon.com: AORUS FO48U 48" 4K OLED Gaming Monitor, 3840x2160 , 120 Hz Refresh Rate, 1ms Response Time (GTG), 1x Display Port 1.4, 2x HDMI 2.1, 2x USB 3.0, with USB Type-C, Space Audio : Electronics
EX480UZ Product Info | BenQ US
Amazon.com: GIGABYTE M32U 32" 144Hz 4K FreeSync Compatible Gaming Monitor, SS IPS, 3840x2160 Display, 1ms Response Time (MPRT), 1x Display Port 1.4, 2x HDMI 2.1, 3x USB 3.0, 1x USB Type C : Electronics
Slashcam News : Asus: Three first 4K 120Hz monitors with HDMI 2.1!
Acer Introduces Its First HDMI 2.1 Monitor | Tom's Hardware