It is simply not possible to determine the final development cost for a high quality product without knowing the quality ambition level. How much will it cost to build an automobile? How much will it cost to build a house? How much will it cost to educate a child? We can give answers. We can even prove that those answers match the question. But, we have left out the question of the quality of the auto, the house and the education. (You risk getting only what you pay for.)