'They said' that Skyward Sword was the largest (in development terms) and most expensive game they ever made when it came out. Probably didn't cost a whole lot compared to some other games though. I searched around somewhat, but I can't find anything either. Seen people throwing around numbers between 8 and 15 million though. I don't think they made anything since then that beats it anyway (though Mario 3D World could be surprisingly expensive maybe), but Zelda WiiU is sure to.