Let me know what you have via PM, and I may be able to give you an idea of what to ask. I've got over 150 of the darn things - and many were bought in your area (DH and I are originally from the Twin Cities and have lots of relatives there). I'm not interested in buying myself so I'm not going to try to lowball you to get a deal.
Machines go from $20 to $2000 depending on what you have, with most around the $150 mark. You don't get into big bucks unless you've got something truly rare.
Interestingly, some of the vintage Singer attachments are worth more than most vintage machines