When new technology is introduced its very expensive, not because the materials are expensive but because there was so much time put into research, development, testing, and setting up a factory to build the new product. Over time those costs are payed off and prices will drop. The same thing will happen when Campy and Sram release their electric group the first few years they will out cost their highest end mechanical group, and slowly the prices will lower. But the electronic group will always be more than the mechanical because it will be the flagship group getting the top materials, and most focus. Because in order to sell a group to top racer(and people who want bikes like top racers) it needs to be the most advanced you have, and for now that will mean electric shifting.
There is no need to turn this into a tread about the value of electronic vs mechanical. There are many, but this is a question about the cost of the Di2.