I would simply state "That is considered confidential information at my current employer. I would consider an offer in the ballpark of /x/."
2 Things - It shows that you take confidentiality seriously even when looking at leaving your current company (and its likely the policy there anyway). Secondly, it is truthful.
2 Things - It shows that you take confidentiality seriously even when looking at leaving your current company (and its likely the policy there anyway). Secondly, it is truthful.