Its almost never in your (an employee/interviewee) best interest to disclose that. You are essentially saying "here is the minimum I will work for" (and that's usually what you'll get) - let them try to figure that out!

What if you say your {last salary + 50%} is your minimum?

Ah I see. I still don't see the point of playing it stupid instead of answer with something like "what were you thinking of?"

"First, can you tell me what you have in mind? After all, you have a better idea exactly what you need done, and your company regularly hires people and possesses an HR department dedicated to that sort of research and negotiation. I, on the other hand, am an individual who--because I'm very good at what I do--am seldom on the market."

I haven't tried that one yet, but I find it's still handy to think about as a way to reassure myself that refusing to be the first to name a dollar-amount (baaaad idea) is reasonable.