I agree with the others - although in my area, I might go a little higher - $25 or 30. The decals are in pretty poor condition and it doesn't look like they're just hiding under grime - it looks like they've been worn away.
It's also a back-clamping attachment and it can be expensive to buy the feet and attachments that make it a more versatile machine. Perhaps as Prism mentioned, the feet are hiding in the storage compartment. If the machine does have a full kit of attachments, I would probably pay $50 for it, even knowing that I would almost certainly have to replace all the electrical components.
People don't usually have any idea of what to charge for these old sewing machines and sometimes they seem to be under the impression that the pretty colors or just their age make them worth their weight in gold.