There is another way. There is quality cad programs for very good prices. IMSI has a product called TURBOCAD, and you can use the basic version, no need for the proffesional version. You'd most propably be able to get a package at less than a $100. With this cad package, you can do amazing stuff. In your case, scanned image saved as jpg or bmp or a lot of other formats. Import the pic in TURBOCAD and trace the image in the default layer with a polyline. Make sure the polyline is "closed" at the end of the trace, and shazzam, export the dxf.
Whacko for TURBOCAD