For the cost of living thing, ideally you just implement similar good urban planning across the country. The reason some places are so expensive is because they have relatively livable cities compared to most of the country, so people want to move their. If you just improve the cities in places people already want to be for some reason or another, then you’ll just get more people across the country interested in being there unless they have similar options near them. Guess you could alternatively make enough housing for like 50 million people in that one city. Technically, there’s always the excuse of “you just didn’t build enough”. Not sure how the cost per housing unit gets for super structures, particularly since the cost of them includes infrastructure costs we don’t usually value into the cost of the home (pipes, roads, etc) and commercial spaces + residential which would make a small city with huge population possible.