Perhaps you should do some material research on the wire/jacket combinations. I think you will find that metal elements conductivity do not really rank by the base cost of the metal. As you have already found, pure metal never does as well as combinations, the same is true as for the jacket. What is true for all combinations is that the shorter the cable the better. Excluding the actual materials cost (metal and jacket), production cost set up usually start at about $15K for a single run. So, you are looking at a rather costly undertaken. There are many companies that could do the job, some of which can do military spec fulfillment.