[chandley]

Dead Monster Bits, DF 2, p. 13 gives the rules for how much various bits of monster are worth. This abstracts what the valuable grue is, I would subsume horns into that.

If it is edible at all, about half it's weight can be turned into "edible" stuff (that is a rule of thumb from hunting, adjust as you like). Less the weight of whatever valuable bits. Cooking rolls should be mandatory to keep from getting something icky.