"Apple in its infinite wisdom and cost cutting measures has now fused the glass to the display. So it only comes as a complete unit. Here's how it's replaced: iMac Intel 27"" Retina 5K Display Display Replacement
Apple Part #661-00200
Reports have been posted of some that have separated the glass but none repaired successfully.
Most units available in the USA cost around $500. A China seller on eBay has it for $310 + $65 shipping. Warning, check the ratings on these sellers."
For small cracks (as long as there’s no missing piece of glass), I used windshield repair kit for cars, and it made the crack almost invisible. Costs about 10 bucks.