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.