Interesting question... I don't think this info is publicly available. I'd guess X4 as well for one of the highest budget or maybe Megaman legends. In real money terms of course. If we're talking in nominal terms, it's most likely Megaman 11.is it known which mega man game had the largest budget? for a long time i sort of automatically assumed it was mega man x4, as both the quality and quantity of the animated sequences led me to believe capcom put more resources into mega man x4 compared to titles like x3 or x5
but that is really just guesswork on my part