The cabinet is in good shape so that is what I would be paying the money for as I believe other machines still fit in that size. The machine is after Singer started using nylon gears in the internals I believe. It may still work but not worth any money in my book as don't know how long it would last. I would take off the top and look in to see if you can see if any of the gears are cracked.
I would offer them $30 and see what they say.