I believe this might be true, but is there actual information to support this claim? The reasons why games might be more expensive to make, like the increasing art fidelity and graphics and physics complexity, overall scope of content and so on are somewhat obvious.

But there are reason it might be getting cheaper to make as well, such as more mature and easily available tools like Unreal Engine and Unity that work on more platforms than ever before, and ease of poring to more platforms due to their increasing maturity and similarity (x86 and ARM, with mature and well documented graphics APIs). Big publishers are also increasingly creating secondary studios in countries with a lower cost of living in an effort to reduce wage costs (such as Ubisoft's Singapore offshoot).

I'd be interested to see some studies or data that show how the cost of development has changed over time as I feel like there is currently only a lot of speculation and assumptions being made here, by myself included.