First up, welcome!
Actually, I have to stand against the belief that spriting costs more than modeling. Spriting, specially for recent sprites, is just a matter of drawing frame-by-frame actions (And some people can cheat their way out of their many animation details). Why I say modeling (mainly compared to iM@S) is more expensive is that you even if they use only one mold all of the cast, to change it to match a character's hairstyle, height, three-sizes, etc... would take time (models of that variety could have over 1,000,000+ polys. And then there's skinning and the clothes. I'm sorry. I'm being a nerd again (And I only heard of most of these stuff. An actual modeler might express this in better and more painful detail).
But I can say the reverse about animation vs. 3D. Animation involves, technically, drawing something over and over and over again until you get to the point where a cartoon is made when all those pictures are played in a sequence. It's why I see more MMD videos on youtube of Miku than I do flowing animations of her. Whenever there is a 2D fan video made for her, it's usually more of a slideshow of timed pictures than an animation.
With 3D animation, once you've made the model, all you have to do is tell that model what to do and make it do it as many times as you need it to, to the point where it probably becomes more profitable than 2D animating. On top of that, I don't think modeling is done little detail by detail like animation. Maybe for specific parts like hair design, clothing design, eyes, etc. But not for basic model parts, so to speak. For example, it's not going to be difficult to change the breast size of a character. There's likely a mechanic already made to simply and proportionally reduce or increase bust size. Same thing goes for general height of the character (which encompasses other body parts like arms and legs).
http://www.youtube.com/watch?v=8yLQp4ieptkI'm guessing editing a character model's size is only a little more difficult or possibly just as difficult as adjusting the size of the dance model in the video, do you know what I mean? It's just a matter of adjusting it all proportionally. Another thing to consider is how detailed you're gonna go with the graphics. Is it Idolm@ster graphics, Atelier Rorona graphics, Final Fantasy graphics? If you compared FF to 2D animation, then I can more easily believe that that 3D animation is more expensive and time consuming. Idolm@ster? I'm more reluctant to believe.
Of course making and adjusting models will take time and effort, but how much compared to animating? Honestly, I don't know. I've never really tried animating or making and moving 3D models. Everything I said is just conjecture based on what I've observed. Ha ha. Who knows, maybe the two forms of art are just equally difficult in general. Unless we can get an animator and modeler to give their two cents, then it's not going to be clear.
Oh, and thanks and pardon me, I don't actually know what I'm talking about. lol
