But how is this sustainable? It's not like paying $5000 per feature means you'll be refunded if prompting "make no mistakes" didn't work.

The only reason why I pay $200 is because LLM's errors costs me that much, at worst. If "make no error" starts working - sure. But surely, unless you have millions of dollars of cash to burn, a coin flip that costs $5000 is an insane idea?