They'll probably be offended. If they ask, just answer, because the kind of people who ask that question are generally not in the mood for mind games. Direct questions deserve direct answers. If you're looking for a salary jump, just say that, and be prepared to justify it. If you can't back it up, you were just bluffing anyway. Don't base your negotiating strategy on a bluff.
It's not a mind game. It's a simple truth. It is totally irrelevant.
Asking is it's own mind game, because they're not asking in good faith, they're asking so they can bully you into accepting a lesser offer. They either don't know what is fair OR they do but want to go "Well we don't pay more then 10% of what you're already on" which has NOTHING to do with your value.
It's a complex truth. Irrelevant or not, you know that whatever figure you give is likely to be what they offer. So you'd just answer if it was the figure you want. Refusing to answer signals that you're asking for more than you used to make. And it's a pretty clear signal. It's not like they'll just go, "Hey, I guess you're right, wonder why I asked in the first place?"
They've already got it in their minds that you're supposed to get the same old salary. It's not so much bullying as just a different perspective on the world. A mindset. You can try to be that guy who finally shows them the light, or you can look at your options in a pragmatic sense. And creating conflict with the guy interviewing you is not super pragmatic.
Edit: One other point. Something is worth what people will pay, by definition. If someone offers you less than you're worth, then by definition you have a better option. So go take the better option. If everyone is offering you the same figure, then by definition that's what you're worth.
Why not just ask what you want, then? If you're leaving somewhere, there's probably a good chance it's about money. Even if it's not, the only reason to ask in this fashion is because every now and then you get someone who low balls themselves.
Otherwise saying "What do you want?" is just as useful without the judgmental overtones of not looking above your station.
If you feel like you're undervalued by 50%, it might be easiest to get a 10-15% bump at a new employer every few years until you're finally caught up. Getting one employer to go for the 50% increase overnight seems significantly harder to arrange.
People who study negotiation have this concept called the “anchoring effect”—the first number that gets thrown out becomes very prominent in the other party’s mind and affects their view of the deal. If I’m on the “employee” side of the negotiation, I want that first number to be as high as I can reasonably justify.
