Additional unknown variables were the development costs and the per-copy costs to build the 7E7. Boeing’s board was anxious to minimize those costs. The forecast assumes $8 billion for development costs; however, analyst estimates were in the $6 billion to $10 billion range. The cost to manufacture the 7E7 was also subject to great uncertainty. On the one hand, engineers were challenged to build a mid-size aircraft with long-range capabilities. The engineering design to achieve this could push building costs up significantly. Conversely, if Boeing succeeded in using composite materials, which required a fraction of the normal assembly time, then construction costs would be lower. Consistent with Boeing’s history, the base case assumes 80% as the percentage of cost of goods sold to sales. As shown in Exhibit 9, however, the IRR of the 7E7 was very sensitive to keeping production costs low.