I've used this as a chance to turn things back around on the recruiters. "That's a great question and I noticed you didn't mention a salary range on the posting". Allow for an uncomfortable silence as now they're either forced to give a range, or try to say something like, "it's flexible for the right candidate". The latter is my opportunity to agree, "Of course, let's concentrate on that question then, I'd sure like if we could get to the bottom of this 'right candidate' question".