What is the trade-in value of a late 2015 iMac?

Go to > Apple Trade In - Apple > click "Find your trade-in value" and enter the requested info.

I suspect that the 2015 iMac will fetch more by selling it on the open market than as a trade-in.

When you are ready to trade-in, sell or recycle,

see > What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

From Apple? A decade-old Intel? They’ll recycle it for free.

Other options? Sell it privately. Or recycle it.

Auction sites list roughly USD$150 to USD$250, plus some outliers.

Related: What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

Considering it is now 10 years old its trade in value is very likely $0.00.

IMHO it would be wise to recycle using one of Apple’s recycling programs. They have a program for various countries, we just don’t know where you are located.

See here:

Apple Trade In - Apple