You can buy IPS 4k display for $599 or less. Not 5k, but not that far off either.
Edit: Downvotes, seriously? Have you seen these side by side? When it comes to resolution, at least I can't tell a difference anymore at a reasonable viewing distance. Display tech simply good enough now.
5k is almost double the pixel count (75%-ish more). It's pretty substantial... also, more to the point with Apple products, I think at the 27" size, 5k is necessary for the pixel-doubling high-DPI option to look good. At 4k, everything looks either tiny or huge, depending on whether you have it turned off or on.