Honestly, for $15, without knowing anything about your mono black deck, I am gonna give Wurmcoil Engine a "meh." Don't get me wrong, it is the kind of card that is really good in just about any EDH deck. However, just because it is good, it might not fit the theme of certain decks. In a deck like Animar, where it can be cast for literally nothing, or Glissa, the Traitor where it can be recurred over and over, it really shines, and is probably worth the money. But there are also other cards for other decks where Wurmcoil's abilities aren't abused as much, that there are other cards which are just as impactful, but cheaper. So depending on your deck, and how you value the card, it is your call.