How can I get a higher salary offer?

I have gone through a couple interviews for a job and I think they are interested in me. During my first interview I was asked about salary expectations....I currently make 50K and told them i was looking for something "higher" with a minimum of a 2K salary increase. Now during my last interview the interviewer suggested if I would take the job if they gave me the 52K, but I was actually looking for a much higher salary offer of around 57K. They haven't officially given me an offer but how can I get them to budge even though I gave them a low number? Also, my current job pays for 100% of my benefits while this new job doesn't. I am also up for a promotion this year so I am weary about joining this job for a 2K raise while my current job promotion will probably get me around the 57K I'm looking for.