According to the Apple trade-in page, how much you get "depends on the device, model, manufacturer, and condition."
"Estimated trade-in value" is "Up to $450" for trading in a MacBook Air. A footnote says "Trade-in values will vary based on the condition, year, and configuration of your eligible trade-in device. Not all devices are eligible for credit." The page won't give you a more specific estimate unless you enter the serial number of your device, then answer questions about it.
I did that once for an old Mac that was in good working condition and the site told me that Apple wouldn't give me any trade-in credit for it, but that I could send it to them and they would recycle it for free …