Oh, you are right, apologies. You can get a pretty nice 27" 4K IPS monitor for less than $250, but not from Apple. Would probably depend on whether your eyes are good enough to tell the difference between 163 ppi and Apple's practically proprietary 218.
I think the old 27" iMac may have used the same screen as the current Studio Display, but the current iMac has a smaller and lower-resolution display.