I completely understand this situation since I been through it before. Here's where your homework and you becoming the interviewer come into play. Tell them what you make now and where that compares with your market. Then follow up by asking them about the duties of the position and the motivation of the company. Are they looking to hire on the cheap? Do they see this as a position of growth?

Don't be afraid of lowball offers. Negotiations and fact finding has to start somewhere. Just because the number is low, doesn't mean you have to accept it. Especially if you're doing your homework and being upfront with the recruiter, headhunter or HR person.