3D models vs 2D sprites: what's more expensive?

It really comes down to the game, but 2d tends to have pretty linear costs for quantity, and exponential cost for quality, and you can see what you're getting pretty quickly.

3D stuff you really need to put a lot of work into modeling, texturing, rigging, etc before you start really "seeing" it. However once you have a nice rig, even with a bad animator, your screen shots will still look nice. I'm not going to say 3D animation is easier, but after a certain point it definitely is faster.

Basically, you get what you pay for, and that goes double for 2D animation.

On most small projects, 2D assets will probably be cheaper.

If you can find a talented 2D animator that can realize your vision either 3D or 2D will seem cheap. I would make the 2D/3D choice based on the mechanics your design needs. A talented animator is worth every penny either way thought since a video game is nothing unless it moves and that movement has to mean something, no matter what game you make.

I can't say much about actually paying someone to do it, but depending on the requirements, if you are doing it yourself, it ends up cheaper to do it in 3d rendered sprites.

This is assuming that your sprites are big enough to enjoy the detail of 3d renders, say 64x64, as 32x32 would end up too fuzzy in most cases. If you have many animation frames, and/or if you are needing different angles like in your typical iso-metric viewed game, 3d starts to look cheaper. Also, if you aren't too skilled and are likely to be making changes, 3d is much cheaper because if you have already drawn many frames with pixel methods, and need to change, depending on that change you have to do it all over all the frames, but with 3d, you do not. Basically, with 2d, the cost is linear, depending on how many frames of animation and how many views. But with 3d renders, the cost curve(as in doing it yourself atleast) is much different. The startup is much longer to be able to get a viable model. Depending on the detail level, you may or may not want to texture it, but if it isn't too detailed, you could just use vertex colors to get what you need, saving UVmapping and texturing time. If the renders will be bigger, like 128x128, you may want to do more detail, including possibly either a normal map, or simply sculpting and rendering the full sculpt if it isn't too triangle heavy, and this process will add to the start time. Then, you have to rig and animate the thing if it needs animation, though in some cases you'd only need some simple vertex morphs.

But, once all of that startup is done, it is very cheap to get your images out. You can easily set camera and render settings, including ortho projection, and render size, and anti-aliasing, and in most programs, maybe an edge render if you need it. And then pretty much instantly(depending on the detail of the mesh among other things) you get your images. If you decide you need different sizes, you simply change and re-render. If you want to change the light position or settings, same thing. If you need a higher angle....well, you get the idea. This also applies if you need to make changes to the model itself, as you only change the model, and then re-render everything.

But, this method is really only viable if you are doing big enough sprites to be able to get enough detail instead of fuzz. Also, it isn't something you need to consider much if you are out-sourcing your art, as then you are dictating the style, not producing the work.

Me...I don't have the money for that, so I do it myself. But I love how in 3d software like Blender you can do things that wouldn't work if you were actually using the models for a 3d game. For example, you can use generated textures, like Blender's clouds, etc... and you can use them with generated UVs as well if you don't want to unwrap the model, for example to create some grainy bumps or something similar via either the material settings or even a displacement modifier. You can use vertex morph animation, which for exporting to 3d engines doesn't work in most engines. You can also use all of the other animation tools and things that for 3d games you'd have to avoid, like not baking IK animation, some modifiers, and many other things. Lastly, one of my favorites is the fact that you can animate pretty much anything in Blender. This includes materials, colors, lights, render settings(some anyway) and even texture settings, like cloud settings and things like that. I once was creating a "reactor" animation, for a missile command style game. It was kind of like a sci-fi reactor, with a screw shape thing in the middle, a base, and a dome for the top. I made the animation a simple vertex animation, which in the game only happened when it was hit, it would "compress" so the middle section disappears, and the dome connects with the base. The material also changes. Normally, there is a light in the middle, which illuminates the base and dome some, and is pretty bright. When the compression starts, the light suddenly dies, and then the dome and base change color slowly as the compression happens. All this would not have been near as easy if I was sending this to a 3d game, but for renders used as sprites, it was great.