If you know how much someone may make per hour you can make a rough estimate on how much you'll need. Or you can guess. I'd at least do a little more then minimum wage or how much you'd like to make if you were the artist.
Let's say it may take 20 hours to storyboard it, 20 hours to make the animatic, 20 hours for backgrounds, 40 hours to animate and 40 hours to clean up. Then you can calculate a rough price.
Of course it all depends on what you want them to do, they may need more to get music, sound effects, voice over, write a script, character/prop designs and composite it.
A tip would be whatever you think it may cost or how long it may take to make it, double that. Most freelancers tend to do a flat rate or day rate though.