As always, it depends.
Game art is a very deep field, so in my opinion you should find a well rounded artist partner to help you, instead of hiring people to do do specific jobs.
Regarding 2D vs 3D, in fact, as Quacks says, creating a 3D model is much more complex than creating a 2D drawing, and therefore more expensive.
However, animating in 2D is increasingly more complex than in 3D as the amount of frames increases, because you have to make a complete new drawing for each frame, while in 3D you it is much simpler. Also, making changes, and creating variations of existing data is easier in 3D than it is in 2D.
In fact, I'd dare say that a very compelling reason why fully animated 2D games are not as common today as they were say, in the 90s, is that 2D is simply too expensive.
However, this also greatly depends on the game you're making. Graphical data for an action platform game is probably cheaper in 3D, while for a puzzle game it may be cheaper in 2D.
Nevertheless, remember that you will need 2D assets regardless of whether your game is 2D or 3D, for the menus, GUI, web/box art among many others.
Also, keeping all of the art consistent and interesting is a very complex task, which is why I recommend you to get a well rounded art director, or become one yourself.
Regarding costs, the skills associated with making 2D graphics are vastly different from those required for 3D, and there are no standards in pricings, so your best bet is probably to ask for estimates. However, knowing what to ask for is difficult by itself, which is yet one more reason to get yourself an art director.