Hey Jaime, do you know what model it is? Can you post a picture? No one can tell you value, so much depends on your location and the supply there and who wants one. Then for the machine itself it's condition (mechanical and cosmetic), case or cabinet, attachments and manual, ect.
I have my grandmother's 1928 66-6, it is an AC number. If it wasn't my grandmother's, I wouldn't pay $20 for it, it was stuck solid and had nothing with it, and it is in a 50's era case. I was able to research and buy the correct attachment set and manual on ebay. It takes someone who appreciates the craftsmanship of these vintage machines to want them. They are usually easily self-maintained and keep working for many generations.
I'm sure you can locate your missing manual and bobbins are available from Sew-Classic or other online suppliers. Oil her up and try her out!
Kim