Due diligence payment

Hi we are in talks with an investor and everything went well, we have now been told that they need due diligence checks which are standard procedures, the only thing is they have said that it costs, and asked us to pay towards the checks. They have said it costs over 10k but we only have to contribute 2k, is this common practice?

They have said that if they do not produce a term sheet offer, then the amount is returned. Hope you can shed some light on the situation.
Thanks