anything can have value- if it's what you want- value is relative-
if you are in the market for a vintage machine you need to set a budget---what it's worth to you- and the conditions---it has to be running- or it has to be (fixable) what ever---then look until you find what fits into your (ideal)
some people will pay a whole lot for an old vintage machine- some won't---it is all a personal choice-
you should start with looking up the specific machine you are considering- check ebay and places like that to see if anyone is selling them- and what they are (trying) to get for them-
check vintage machine shops-
check antique shops-
a singer featherweight can go for anywhere from $25===right on up to $500+...it all depends on what it is worth to you.