Key date coins are rarer than others dates due to how many were minted.
If your coin was in mint state or mint state +, meaning looking like it just came from the mint, or uncirculated, MS+ is the highest rated mint state, it would be worth from $100 to ten of thousands of $ depending on it's grade, starting with a MS-4 ($100) and the highest grade of MS-70, I don't think there are any known 1915 halfs that grade above MS-66 or MS-67, the highest value I've seen is a MS+ 66 valued at $41,500, whereas a MS 66 would be valued at $19,500.