I'm guessing you're talking about a sewing machine?
Your best bet is to look on ebay, craigslist, etc and see what other Damascus Grant or similar machines in a similar condition are being sold for.
I will say, however, that things are only worth what someone is willing to pay for them. My MIL gave me some framed artwork several years ago, told me what she paid for it 30 years ago and that I should sell it for double that. I was scratching my head wondering why *she* doesn't sell it, she needs the money more than we do. I tried to sell it and got no takers. Had it appraised for insurance, paid more for the appraisal than the art was worth.