Unless it is absolutely pristine like the ones on Graham Forsdyke's website (
http://www.singer-featherweight.com/index.html), $600 is too much. Even if all the accessories are there,you need to make sure the foot pedal has been replaced or any changes have been made to the machine. Also, how old is the machine?
It is difficult to really check what the condition of the machine is from the one photo. I agree with TexasGurl that maybe $400-450 should be the max.
The machine should only be higher than that if the machine and case are absolutely pristine with as much of the original attachments and equipment, or it is the FreeArm version (Singer 222).