If it is a Marlin in good shape it is worth right at $1000, if it is a Big Bore 94, then it is valued between $600 and $800.  
I have one and have owned a couple in this offering in my life.  Like I said, it is a good cartridge and the only thing besting it by enough to mean anything is the 338 Marlin.  The Big Bore action has the extra thick receiver, but I think almost all of the 356's have the safety wart and the rebounding hammer.  From what I understand, there were some Angle Eject 356's made without them but I have not seen one.  One of those models would be worth a lot more I would think.  Box and papers matter so remember to ask if he has them.  
$600 is a good deal if the rifle is in good condition overall, if it is a Marlin it is a steal.