Welcome to the site. Not having seen your piece of walnut it's hard to nail down a price. Based on what I know, I'd guess it is worth somewhere between $100 and $200. If it were longer it would sell better cause it could be used to make a mantel. The amount of sap wood, checks, number of knot's, knot placement and figure all would have to be entered into calculating the price. Also, the buyers need for a block like the one you have. Might be worth more to a sculpture than a turner or vise-versa based on grain patterns within the piece. Hey, it might be worth more to use it for what you had in mind originally.