Maybe, it's all extremely fluky tbh. Even if you use what little we know from other games or from journalists. I'd be hesitant to guess that a Japanese 3rd party is spending as much as Sony on a western first party game in the current era. The latter should be more expensive, as it can bear the weight since they don't give up anything in revenue sharing. My paper math on it would look something like:
200-300 full time devs working for ~5 years, probably longer due to COVID. IIRC we've previously been told that it's ~$100000 per head per year in the west, but we know it's lower in Japan, so let's say $80,000, and split the difference on the other numbers, say 250 and 5.5?
So that's 100-120m there. Of course that doesn't cover marketing/distribution, along with contract/outsourcing costs, VA, etc. Which I'd assume would easily be another 100m. Maybe undershooting there, but on the other hand the game hasn't been relentlessly promoted like you might seen on other big marketing cycles. ~250m seems more likely to me? Of which a portion is covered by Sony. Really this is all pretty weak guesswork though.
Why do you think the game needs to sell 8.6m to break even at $300m btw? Seems like they'd probably be there at ~5-6m given that the early sales would heavily skewed to full price digital, and the more expensive collectors physical and digital editions.
Honestly I can't imagine the break even point for FFXVI being dramatically different from XV, which was ~5m at full price apparently. Obviously budgets have grown since then, but XVI is an exclusive and has a base $70 price point, so that should mitigate the difference.