If I have learned anything about vintage sewing machines it would be that pricing is bizarre. I think when you mix in sentimental value it mucks things up. You have people that have no idea what to charge and that mucks things up. Shows like Storage Wars that pull vintage items out and claim they are more valuable than gold. etc etc.
Mostly I think people like us are an uncommon breed, I won't say rare because obviously other people are buying machines too. Maybe to use, maybe to display, maybe because it reminds them of grandma. Most of the machines I have purchased I intend on cleaning up and using. But a lot of people out there seem to assume no one is using these machines.
I'm babbling. I suppose if you want it offer what you are comfortable with and if they aren't interested move on. Unless it's OMG RARE (which this isn't) there will be more. I'm with Joe, tell them you are interested for $50, never know, in a week that $50 might sound like a really good offer.