Apple does have a trade-in scheme. You can check eligibility - and obtain an estimated trade-in value - here:
Apple Trade In - Apple
The available trade-in value that you see listed in the online trade-in calculator is the maximum estimated value - and assumes that the iPad is undamaged and fully working. The actual value is only finalised when the qualifying device has been inspected and accepted.
Be aware that online and in-store trade-in values can differ - however, many find that completing a trade-in using the in-store process removes any doubt as to the value offered; the transaction is finalised without the risk of shipments going astray, or disagreement as to the condition of the device when it was sent to Apple’s trade-in partner.