Has the cost of building software dropped 90%?

It depends. For AI to work for large projects (did a post on this forever ago in AI terms. https://getstream.io/blog/cursor-ai-large-projects/)

But you need: a staff level engineer to guide it, great standardization and testing best practices. And yes in that situation you can go 10-50x faster. Many teams/products are not in that environment though.

I work on a big ball of open source spaghetti and AI has become invaluable in helping me navigate my way through it. Even when it's wrong - it gives me valuable clues.