Very tough to give an estimate without at least a picture. Most old sewing machines from that era run from free to 50 or 75 bucks, Many of these machines are in serious need to lubrication and, in my experience, can take some work to get freed up and running right.
Kenmore 158's however, can be an outlier. There were bunches and bunches of variations. The 158.1941's I see go for a hundred bucks and more for reasons I haven't figured out. All the 158.xxxxxx (varying number of decimal places) series machines I've looked at or researched have a double pulley system internally that increases torque and punching power. The free arm versions seem to be the ones people look for.
One of my daily drivers is a 10-stitch, 385 series Kenmore from the same era with the pulley reduction and free arm.