So give them your salary. What is the point of wasting your time and their time if they can't meet your salary requriements. They can afford you or they can't.
We had this happen at my company last week. A caniddate said he was looking for x amount which was within our range. We interviewed him and extended an offer that 10% higher than what he was asking for (we asked what he was looking for not his current salary).
He countered and said that what we offered was below his current salary and that what he really wanted was x amount. That amount was way above our salary range so we said goodbye. He said he gave us the lower number becuase he was afraid we wouldn't inteview him if we knew the truth, and he is right. Well all he did was to reach the same conclusion but waste everyone's time.
Just because you interview well does doesn't mean a company will go outside of their budget or salary range to get you. That is why i say be honest, if they can't afford you , they can't afford you and a good interview won't change that. It will do nothing but waste your time.