Has anyone got the trade in price quoted for an Apple product?

If you use the trade in service, Apple does not do Macs in store (only iDevices); you do need to send it to a third party. They will determine the final offer, so take lots of photos inside and out and keep track of the tracking number when you send it so you can check on its progress. If you are told that it is damaged, tell them you have proof that it was in pristine condition and offer to send them the pics.

I received the value listed multiple times. Some folks just expect the full value and turn in a dented device that doesn't function properly.

Put it up for sale locally. You will get much more than any trade in could possibly give you.

When I got the offer from the bunch Apple uses, it was an insulting 75% less that what the original quote was and they gave no reason for the decrease. This was a 2015 21.5" Retina screen iMac. The problem with the computer was it had become dismally slow over the years. I fixed that with an external SSD boot drive. After that it ran [by the numbers] 5x write speed of the slow internal HDD. I later replaced the internal HDD with an SSD and write speed went to 6x original HDD write speed. It's now my backup iMac and our AirPlay Mac and has been operating flawlessly since January of this year. YMMV.

If they do not give what you believe is fair then cancel and they will return it to you and you can try to sell via other routes (e.g. eBay, etc.)

Any older stock 21.5" is dismally slow because of its inadequate 5400 rpm stock spinning drive. I would not be able to work with that, even out of the box.