I should mention - how I work it out is how a shop would value a machine - which is why they offer nothing for trade in. Once sentimentality gets involved, on either side - the purchase price WILL go up.
Almost all of the machines I buy are with the thought that I may one day change my mind and decide I want something else / out of sewing machines altogether / etc. So my goal is to force appreciate things and to not get into anything too far that I can't get at least some of my money back out. Someone strictly collecting will look at it differently.