You question is all relative and broad. I have seen make motion graphics made for $5 dollars on fiver, for $500 from by a graduate student to tens of thousands of dollars when using an agency or an artists that is high in demand.
The real question should be what is it that we need, when is our deadline and what is out budget. This can help tremendously.
You also have choices as well and they each have their own budget ranges. For example there are Mograph templates companies that make animation templates so we can do the animation ourselves. Having a budget helps in choosing who will be the best fit for our project and also helps the artist we hire get a better idea of what they will and will not be able to accomplish for us. This information can also help save time. :)
Also, if there is an video animation we love and would want a similar style we could find out who produced it and contact them directly.
For more information you can visit here: Motion Graphic Video Cost