Ask them for enough to cover the cost of the hair. That way if they change their mind about fitment then you're not out of pocket. Explain during the consultation that once you've ordered the hair it cannot be changed and the deposit for the hair is non refundable. If you don't feel you can do that then I'd definitely consider asking for no less than 50%. What you have to take in to consideration is that you may be left with hair that will be absolutely useless to you.