The wiring you described is very basic and a pretty easy fix. An original, vintage bobbin case will cost you up to $50 plus shipping. Bobbin case copies are less, but not as reliable as they're made in China. If it were me, I'd feel comfortable offering $100 but I would tackle the wiring myself no problem. If you have to pay someone $50-$75 to do it for you, you'll have to ponder if it's worth it to you.