Here's an easy to understand example. I've been playing EvE Online and it has an API with which you can query the game to find information on its items and market (as well as several other unrelated things).

It seems like a prime example for which to use AI to quickly generate the code. You create the base project and give it the data structures and calls, and it quickly spits out a solution. Everything is great so far.

Then you want to implement some market trading, so you need to calculate opportunities from the market orders vs their buy/sell prices vs unit price vs orders per day etc. You add that to the AI spec and it easily creates a working solution for you. Unfortunately once you run it it takes about 24 hours to update, making it near worthless.

The code it created was very cheap, but also extremely problematic. It made no consideration for future usage, so everything from the data layer to the frontend has issues that you're going to be fighting against. Sure, you can refine the prompts to tell it to start modifying code, but soon you're going to be sitting with more dead code than actual useful lines, and it will trip up along the way with so many more issues that you will have to fix.

In the end it turns out that that code wasn't cheap at all and you needed to spend just as much time as you would have with "expensive code". Even worse, the end product is nearly still just as terrible as the starting product, so none of that investment gave any appreciable results.