How bad do you want this job? If not bad enough, say this to the HR:

"I appreciate the offer but I strongly believe that my market value should get me $desired and if you cannot match that, I will not be able to move forward."

Hit send, close tab and go away from your emails for a while. If they truly value your skills since they already interviewed you, you will get the deal. If not, hey you didnt lose anything

This has worked for me many times.