What you need to do is spell out EXACTLY what they will get from you (length of animation; basic items in the scene; camera movement, etc.) and charge a flat rate based on how much time you think it will take you. You may have to alter the initial price if they come back to you with a more detailed list of what they expect. Send them the revised estimate based on what they now expect. Then....   when they decide to add and make changes to what you already have, charge an hourly rate that you feel comfortable with - be that $10/hour or $50/hour or $100/hour or anywhere in between.

This way both parties understand what is expected - you don't work for free and they receive what is spelled out. Inevitably clients request changes, but if you charge one flat rate and they expect changes within that flat rate, you will be shortchanging youself and they will expect the same treatment next time.

This is what we do. We give them the estimated price of the project and our hourly rate is $75 for any changes they require. Oftentimes the changes are minimal and we don't charge, but when they start a list they already know that it will cost them more and we have yet to have a problem after over five years.

I would also recommend you don't charge too low an hourly rate. Why? If this is a respectable company and you charge them $10/hour, they may not think quite as highly of you as someone who charges them $50/hour. Why? "This guy can't be that good if he only charges $10/hour..."  It all depends on who you are dealing with.

Hope this helps.

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