It this price disparity will only last as far as the demand for these areas stays low. As the demand from remote workers fleeing large cities grows, so will the prices. See Austin for an illustration of rapid gentrification due to migration.
It takes decades to build out another such city from scratch. I'm not arguing that it shouldn't be done, just that it doesn't happen quickly.
Maybe time for a Kickstarter for cities?
If the outflow from the really big cities was equally distributed (say, as percentage of city's population), then your view would be accurate. But that's not what's happening.