How much is this subwoofer worth?

Try hifishark: https://www.hifishark.com/search?q=Rockford+Fosgate+t212d4
Search expired if nothing is for sale,now.

Check the "Sold" listings on HIfiShark.com. Click on the "Sold/Expired" tab and then on the "Site" label at the top of the listings to narrow down the results to specific geographic areas.
https://www.hifishark.com/search?q=Rockford+Fosgate+t212d4

hilde45 and I had the same suggestion at the same time.

It's for a car apparently.  One on eBay for $375.

I wouldn't go more then three fitty. https://youtu.be/c4-KrTJVHFs?t=47

The value of those older car audio subs are primarily driven by condition and rarity.  These aren’t very hard to come by but built like tanks.  You never stated what kind of shape yours is in but if its ‘new in box’ it could be quite valuable.  If it’s experienced a life of playing bass heavy music in the trunk of someone’s car for almost 20 years then it’s probably not going to be worth much.