This is freelancing and there is no set costs around, you have to under cut your competition.

I would look it as:

Q. How important is your time?
A. Well everyone's time is important, you just need to price it.
Q. Will you spend a full day (8Hrs+) working on it?
A. this would effect if you charged an hourly rate.
Q. The complexity of the animation?
A. Key really, the complex, the longer it takes
Q. What is the deadline?
A. If you don't have a lot of time then, up goes the price.
Q. How long have you been animating?
A. Your skills have price, new guys charge less and the more experienced

Lotsa questions to base your price from.

Look at other freelancers websites to see if they have pricing online, and then make a comparison. It's really your choice. Maybe if the site has a freelancer working who might help you out.