Prices are dependent on area, supply, & condition. If you live in an area where they don't come up very often, prices will be higher. Most are not in good condition; this lowers the price. If one has accessories and is recently serviced, it will be higher. And it all boils down to whatever someone is willing to pay.
I love finding non-working machines, re-habbing them, & finding the missing parts. When I sell them, I can't let them go for $20; I have WAY more than that invested in parts, not to mention the time. I buy the machines to use, not to make a million $. But I let one go to a new home every so often.