Supply and demand, economics' building blocks, may be simple theoretically, but when it comes to housing they can get complicated. Specifically, supply's powerful role as a demand driver is a complex phenomenon. Here, Zillow data analyst Matthew Fix looks at supply in terms of for-sale inventory, divides it into three tiers--bottom, middle, and upper--representing three broad price ranges, and shows what the market looks like. Point is, so little inventory at the bottom tier stunts demand at that level. Is this opportunity or does it represent a risk to housing's tentative recovery momentum?