Business owners who are hoping to sell their business will often ask for a Non-Disclosure Agreement from a prospective buyer before they will pass over their financial statements. The reason for this was discussed in a previous recommendation, but a brief synopsis is that you want to separate the serious buyers from the tire kickers.
The further question addressed in this recommendation is why you want to ask for a payment — a deposit to be sent with the NDA. This is a higher level of concern. If you are about to give your financial statements to one of your competitors, then I would for sure be looking for a payment. Otherwise, you are giving them all the information they would want about your business and you are getting nothing in return.