Okay, so it appears that you have an AlNiCo Sunn 128G Transducer.
My usual response to anyone who asks what something is worth is that it's worth what someone will pay you for it, and what you're willing to accept. When buyer and seller agree, a price is set. Anything other than that is speculation. However, you can often find the prices that other buyers and sellers have agreed upon by looking at ebay completed auctions. IIRC, last I noticed, which was some time ago, they were going for about $25. However, I could be mistaken, or misremembering, or the going rate could have changed significantly.