It is very early days to be arguing about development cost with any degree of specificity as the design itself is not even bedded down. I don't think even the Japanese officials working on this program have an idea of what the number might be. In real life, once the design and initial requirements are defined the engineering estimate on development, on unit cost and in ongoing service like cost could then be costed. I am sure at that stage they will get to that "on my god moment" A number of iterations will then be debated to manage program cost. It will then be taken to the politicians and that is when the fun begins.
In the real world, 90 % of program failures are due to poor execution. There are not many stealth programs out there to benchmark and due to the diversity of them it is difficult to make direct comparison. You need to have access to the cost structure of the program itself to understand the cost drivers. For example, an extract from the work done by RAND on comparing across programs.
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The question is why is there such a big difference between the F-22 and the F-117 program?
I think we need to be patient and make objective assessments progressively as and when details of the F-3 program are revealed. Making broad stroke comments based on a superficial view of the facts is very amateurish.