Is there a base with it, or just the paddles? Book values are based on a complete set.
Cleaning is the same as with any cast iron piece, except you have the handles to deal with, which I assume are wood. On Griswold, they can be removed by extracting the pin holding them in the casting and carefully easing them out. Then you can do lye or electro, or oven cleaner or vinegar, as necessary. You could also refinish the handles while you have them out, or leave them as-is if still in relatively good condition.