If you do it in the correct order, first massively reducing car traffic and then separating it, the costs should be manageable over time. The point isn’t to do it all at once or overnight. This has the “side” effect of making cities a lot more human friendly even before doing anything to separate traffic while still reducing the complexity of navigating the city FSD.

Re-purposing one of the existing lanes (a bus lane can move more people than 3 SOV lanes combined). Toll roads, reinvesting parking revenue, and increased tax revenue from downtown land used for working, living, and playing instead of just storing automobiles.