You can verify eligibility for trade-in and obtain an estimate of the potential trade-in value here:
Apple Trade In - Apple
While trading-in offers an opportunity to recover some of the original purchase cost, you will only receive a fraction of the device's original value. As such, if you are seeking to add cellular connectivity to a WiFi Only iPad, there may be more cost effective alternatives to replacing the iPad.
If you have an iPhone or other mobile phone, you may be able to use the hotspot feature of phone to provide cellular network access:
How to set up a Personal Hotspot on your iPhone or iPad – Apple Support
Alternatively, purchase of a readily available Cellular Broadband Router (often known as a MiFi Router) may be considerably cheaper than replacing the iPad.