The rule of thumb is that it takes 8 hours of work to produce 30 seconds worth of top notch content (this is of course taking into account client feedback).
Rates for freelancers vary but someone good will charge you anywhere between $ 70 per hour and above
So the cheapest you are looking at is $ 1120. I would double this if usable assets are not provided by the client, like rasterized JPEGs instead of high res AI or EPS files (or similar) with transparency.
For more information you can visit here: Motion Graphic Video Cost