I've been discussing budgets and profits with DKII for a while now. He pointed out that it's basically impossible to know the budget of a game because they can be pretty much anything, but he also said that often a lot of the budget comes from the storyboard and art, so an RPG with a lot of both of these things usually has a naturally higher cost of development. He said this is the reason that RPGs tend to not be released until a couple years into the generation. They stall time for userbase to make up for high dev costs.
However, he also said that when a game is developed by one company and published by another, usually the publisher hands the developer money up front that accounts for both dev costs and the developer's payment for the game. Any money made off sales then goes to the publisher, which he said makes about 50% of the retail sale price.
My theory is that in this case, given that Microsoft has basically bottomless pockets for their console in Japan, no expense was spared. The game was developed at a fairly costly level, but Mistwalker was also handsomely compensated for their work, and the only one facing any cost loss here is Microsoft who never cared in the first place about the cost.
Anyway, it's impossible to tell for sure so continuing the debate is basically useless. IMO Mistwalker's games have been very fun, and that makes the developer successful in my eyes, but I don't think the sales are anything to brag about.
Edit: As a side note Legend, DKII also mentioned that your theory doesn't work because of Guitar Hero, which gives them a much larger piece of the pie than a standard retail game, which he said a publisher recieves about a 50% retail sale price return for.