"When a client asks, “can you lower your rate?,” you can respond in various ways. You could get defensive and belligerent (“For your information, I’m a serious professional whose work is worth real money”). You could offer some snarky feedback on the rate the client is proposing (“No serious professional translator would work for what you’re offering”). You could justify why you charge what you do (“I have 20 years’ experience and a Master’s in Translation”). You could just say no, and suggest that they find another translator; fair enough.
But the best response is, “I’m unable to offer a discount, because I’m busy all the time at my regular rates.”