One question I love to know the answer to is why we only built one, or how hard would it be for us to build more now? Would a second one have cost 5%, 10%, 20%

One question I love to know the answer to is why we only built one, or how hard would it be for us to build more now? Would a second one have cost 5%, 10%, 20% more? Surely it wouldn't be near 100% as all of the research and testing scales.