Has the cost of building software dropped 90%?

To nick pick, it is more about 800% gain in productivity. Cost actually increased.

I refer here to my experience as a solo developer.

With AI assistance I don't spend less hours coding, but more.

There is the thrill of shipping relevant features that were sleeping in my drawers for ages, quicker. Each hour of coding delivers just 8 x more features and bug solving.

Also, whereas I spent a few dozen dollars per month on server costs, I now also spend an equivalent amount on subscriptions and API calls to LLM services for this AI assisted coding. Worth every penny.

So while productivity increased manifold, absolute cost actually increased as well for me.