i want to trade in my older imac for a new one. what can it be worth trade in

Depends on the year and condition. Please start https://www.apple.com/shop/trade-in and then you can begin researching what the range is and what yours may be worth.

Another friendly advice. If you decide to trade in your old iMac for new, I seriously recommend taking photos of your iMac before sending it for trade. Had people in this forum complaining about trade in being rejected because of damage. Apparently, no photos taken, nothing can be done.😳