If all is factory original then you could be looking at right around $2500. But there are alot of things to look for to ensure its 100% original.
Most folks don't even know all of the stuff a 1964 SP-1 came manufactured with. The barrel is the first give away. If it even mentions chrome bore or chamber its not original right there. Era correct will only have a "12" or "MP" marking. Then look at the buffer. Not an Edgewater. Ding, Ding, Ding. Again not orginal.
The list goes on for another 5 or 6 items as well.