The best thing I can tell you about monetary value is to watch Craig's List daily for a while. That will at least give you a feel for what people are asking for old machines. You can also try Ebay's old listings to see what machines similar to yours have actually sold for. Some people ask for crazy amounts of money. There are machines on my local Craig's List that have been sitting for literally a year and are still being listed.
I haven't paid over $25 for a machine yet. I've learned if I'm patient the right one will come along at the right price.
Rodney