I am in China and have used an agency once but never did need to pay an up-front fee. That should be between the actual agency and the job provider.
I post because I have a concern. The "entry" visa that you require to teach in China insists that you have at least two years working experience (in any field). I do not want to put a dampener on what I am sure is a very exciting move for you, but I would be very wary of giving money away before you are sure you know what you are doing and who you are dealing with.
I have heard that certain provinces are not so strict on the above detail, indeed private schools are "generally" not so strict also. Just be careful.
Also, be wary of the visa that you need to teach in China. It is a "z" visa. And note that this is an "entry" visa which has an added requirement that you obtain a "residency permit" within 30 days of arrival (after telling the authorities that you have arrived within I think, 48 hours).
Do some more research on this. There is a lot of opportunities to teach here and certainly not just one agency to help you.