how much Could i trade in my ipad air 2 for

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 - amd 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.

The trade-in value is returned to you by way of an Apple gift card for each device - and these can be used to purchase whatever you wish with Apple.  An accepted trade-in will be for credit against an immediate or future purchase - and not for cash.

If a trade-in is initiated during an online purchase, you will usually pay full price for the item - but will receive a refund against your credit card upon subsequent return (and acceptance) of the item that was returned for trade-in.