It's hard to say tbh, but I did find
this PC Gamer article that says that just 300 devs worked on both AC6 and Elden Ring. Presumably most of the staff had worked on Elden Ring consider the bigger scope of the game, while a smaller team worked on AC6. Game dev salaries in Japan are also considerably lower than in the US and probably lower than in some parts of Western Europe like UK and Netherlands. So if we assume a team of 100 devs have been working on it since 2019 when they shipped Sekiro with an avg salary of $52k per year (
The estimated total pay for a Game Developer is ¥7,678,992 per year in the Japan area) that will get us around ~$21M. Let's add an additional $10M for outsourcing some of the work (e.g. asset creation & CGI trailers) to some JP & SEA studios, that would result in a $30M production budget. That said, I'm not sure how big the marketing budget was but overall I'm sure that they got some very good ROI with AC6.
Edit: They did mention that at peak times they had up to 200, 230 developers working on Armored Core 6, so let's consider that this period lasted for around 2 years (probably it was shorter than that as Elden Ring shipped in Feb 2022 while AC6 released in Aug 2023). Taking that into account the numbers change a bit, with an additional $13M expense being added to the production budget I've mentioned above, resulting in a $43M total dev cost for the game..