Broken screen on iMac 27 how much it cost to change it

The price will vary depending on the age of the iMac and whether it is a traditional, 4K, or 5K screen. Get a quote from your nearest retail Apple Store, or Apple Authorized Service Provider. That will be the most accurate quotation.

Cracked 27" iMac Screen Repair Options? … - Apple Community

https://www.amazon.com/Apple-Intel-Front-Replacement-922-9833/dp/B007WGD3H2

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https://www.google.dk/search?q=imac+27%22+screen+repair&ie=&oe=

You have to contact an AASP in your area or you have to contact someone in a neighboring country where Apple either has an Apple Store or AASP. Costs will vary by location, so it may be wise to contact several locations.

It may be expensive as the display will have to be replaced, if it were me I'd anticipate $500USD +.