Old machines turn up in the local charity shop regularly (not all the time, but I see quite a few nice machines a year). In reasonably good condition (yet dirty and in need of care) they are first priced around $180-200. They stand there for a week or to, if not sold price goes down to 100, the good machines are usually sold then. A week later they are often set down to around $70, and treadles are always go withing a day or two at that price. These are usually not complete, mabye with a few bobbins and feet, some have traces of rust, but never terribly much. Domed cased machines usually go for 60 dollars, hand cranks are sometimes more sought after than electrical for some reason. I guess it varies from time to time and place to place, but the prices seem to reasonably predictable. The bargain give aways are often on sites like craiglist or towards the end of a fleamarked weekend or garage sale.