Often times this is a pattern, the person on the other end feels like they need to get a win in some form and the easiest and most obvious to them is always pricing.
Find out why they are asking this question, find out what they really want - 8 out of 10 times for me, it is just them asking because it would be dumb not to ask. Explain your rates, be open and honest about them. If they feel like your rates are too high for them then be willing to negotiate.
It is incredibly important that you negotiate, so many other people I talk to simply balk and let their rates come down some. This is insanity, it is surrender not negotiating. I always, always ask for something in return if I am giving up something and do my best to make sure what I get in return is more valuable than the money discount they are asking for. Often time I use this as a segue with a customer to discuss retainer billing models. Most people balk at it but if it is part of a give and take over the rate they will embrace it. Give me the choice of a project with pre-paid hours at 150/hr vs NET 30 at 165 and I am choosing the 150 all day long.
Another point, if you relax your rate - sunset the change. 'I am willing to do this project at $150/hr but only for this project - future work we do together will revert to the $165/hr rate'. Put it in writing.