To answer your question, here are some factor of assets cost, as a factor of time, skill and quality of the resulting art.
To make sense, shall we quantify that we are talking about a single asset.
A game character.
The pricing will differ greatly, not between 3d & 2d, it will differ between professional experienced artists and skilful amateurs. It will also differ if you decide to use outsourcing.
Now, lets look at 2D vs. 3D, again, the cost will differ a lot based on the amount of animation the character requires, more animation = more work.
That being said, it is generally less time consuming to animate in 3d because of advanced software that offers tools like bones. When animating a 2d character, the artist has to in most cases(unless using cut-out style rotating limbs) render each frame of animation by hand, that is a very time consuming process.
Generally per character, it is nearly completely obvious from a technical perspective that building one 3d character with basic animations, is more expansive than one 2d character in the same 'league' of quality. Why is that?
Well, when building a 3d character, you need to construct a model, create a uv-map, paint a texture, rig the bones and then animate it. Since it is 3d, it has to look good from every angle, so a lot more thought and work has to be put into examining the model from various angles while animating and constructing it. Also, because of the technical aspects of UV-mapping and rigging - bones, certain software related issues could rear their ugly head and the artist will have to face these technical issues.
You can normally only see 2d characters from one angle, so that is worked out, there is no use of advance software need to create classic animation. That said, animating something like King Of Fighters takes great mastery, knowledge and dedication.
So the short answer is yes, 3d is more pricey in terms of work hours. high level 2d animation is also very pricy. The previous two comments are less prevalent than the choice between outsourcing vs. working with someone in the same office and the choice between taking on someone who has proven experience vs. a skilled amateur. The most important variable is the scale of the project.
Also, remember that if you take a 3d-modeler you would probably have to hire a level designer as well, cause a professional character modeler might is not very likely interested to be involved at all in level design. A 2d artist that can design characters well is likely a reliable source for background art too.
I suggest you ask several artists and compare their proposals.