Values are subjective. If it's in demand and there aren't many around, then the value is higher - the old supply and demand thing. Certain sewing machines are more common in some areas than others, so they are worth less. Keep in mind that millions of sewing machines were made in the last 100+ years and not too many people sew these days. So, it depends on where you are and how many of X machine are out there.
For example, take the 411G currently being discussed on this site:
1965 Singer 411G in original treadle spec: ) Rare??. It's unusual to find one in the US, but there is no huge demand for treadle sewing machines, even an unusual one. However, if you took it a meeting of sewing machine geeks, you might get $1,000 for it (Yes, there are such meetings.). If you tried to sell it in Dallas on Craig's list, you would probably be real lucky to get $200. for it.
Generally speaking, working vintage sewing machines are worth somewhere between $25.00 and $300. If you have the attachments and accessories, a manual, a cabinet, and it's in good condition, it will usually add to those amounts.
We might be able to tell you, generally, where on that spectrum your machine would be might fit if we saw a photo.
If you want to know what you have - we can usually identify most machines from a photo - or, give you a category it fits in. That way you can do your own research into it's value where you are.
bkay