But also achievable on a $150/mo (CAD) Max 5 subscription (I currently have 11.6B tokens in the last 30 days) according to /usage. It doesn’t break down input vs. output tokens as far as I can tell.
It's 6B output tokens, as stated by the blog post.
When writing software with Codex 95+% of tokens are cache, I would assume the same in your case (if you also used it for coding).
~10B tokens a month is pretty typical overall input/output usage from my own experience and other developer accounts I've seen