There is no marginal cost for training, just like there's no marginal cost for software. This is why you don't generally use unit economics for analyzing software company breakeven.

The only reason unit economics aren't generally used for software companies is the profit margin is typically 80%+. The cost of posting a Tweet on Twitter/X is close to $0.

Compare the cost of tweeting to the cost of submitting a question to ChatGPT. The fact that ChatGPT rate limits (and now sells additional credits to keep using it after you hit the limit) indicates there are serious unit economic considerations.

We can't think of OpenAI/Anthropic as software businesses. At least from a financial perspective, it's more similar to a company selling compute (e.g. AWS) than a company selling software (e.g. Twitter/X).