This depends on a lot of things! Does the horse have famous blood lines/ successful parent's Is the breed of this stallion more sought after than more standard breeds? It also depends on were you are getting the horse from. Another thing would be if you just want the stud services from this horse, or you if you would actually be buying it. Many people breed horses for fun, and sell them for future riding horses or keep them for themselves so this means at some point they may have a black stallion, but if they do it just for fun, its likely that the stallion will be gelded unless they are going to use it for a future stud for their mares. If the stallion it produced by a serious breeder, its likely to be much better quality therefore more sought after, therefore more expensive! And of course the future potential of the horse will increase or decrease the price. So really, there are to many factors to give and accurate estimate =) Hope it answers your question.