So how could OpenAI make fine-tuning so cheap? Beside the computing power you need for fine-tuning, OpenAI also have to save your fine-tuned weights, and spawn

So how could OpenAI make fine-tuning so cheap? Beside the computing power you need for fine-tuning, OpenAI also have to save your fine-tuned weights, and spawn a new instance for you every time you want to run your fine-tuned model. If so, I imagine it must be insanely expensive because the inference only model of ChatGPT is not what one can say "small". So, how did they do that? Can anyone share their "secret"?