Since I started buying vintage machines I get asked the question on value a lot. I feel the same as Candace. After awhile of this I came up with my very own answer that keeps me in neutral territory to any questions on value:
Your machine will be worth what you get for it the day you sell it.
I answer this way because there are so many variables that affect prices, supply and demand, quality, condition, make, rarity, timing. Sometimes you sell an item just when someone is looking for it, the list goes on and if a prospective buyer really really wants it. I had a woman drive inland from the coast two weeks ago 200 miles total one way over a mountain range with narrow twisty roads in bad weather and spend the night in a hotel in a town 40 minutes away from me and then drive down interstate 35 minutes the next morning to pick up a darling little Necchi cabinet and matching chair for her Necchi. I asked $60.00. She spent a lot in total to get that little cabinet. Must have been a great value to her but it sat on CL for awhile...