Typically, it’s about 1/3 of the selling price, but there are times when a specific device is in demand like the iPads were last year. In November or December 2020, Apple increased the trade-in value for iPad Pros. I jumped on it and traded in my refurb 2018 12.9.
Keep in mind that the trade-in value quoted goes down pretty quickly too. If you look at the trade-in value as soon as your device is eligible to trade, and then look again a month or two months later, it will likely be lower.
You’ll always get more for your device by selling outright on a site like Swappa, or doing trades with reputable vendors like CashForYourMac.com and MacMeAnOffer.com.