Sam Altman has said in a few interviews that it was around $100 million for GPT-3, and higher for GPT-4.

But yes, this is a one-time cost, and far lower than the "millions of dollars per second" in GP comment.

https://fortune.com/2024/04/04/ai-training-costs-how-much-is...

https://www.wired.com/story/openai-ceo-sam-altman-the-age-of...