To define those levels you gave in the example you would need to know the probabilities to start out with. But as you say no one knows those probabilities. And the different contracts would become (un)interesting as new information becomes available and so the community estimate goes up or down. The icbit Dec contract started out at 300M and it last trade was at 484M. This free trading of these future contracts allows the community to bet on the future difficulty without anyone having to guess the probability upfront. It allows the price to reflect news as they become available from the ASIC vendors. And if enough people participate, the efficient market hypothesis predicts that the market value should be a good estimate for the December price. In that case it would not only help miners to hedge but also give them this good estimate as free information. Right now the estimate is probably not very accurate because the volume is low. On the upside, this gives a good opportunity to anyone interested to bet on the difficulty and who think the difficulty will be lower or higher than currently traded.