I suspect nosulc is right, the 260 case is probably made during the same run as the 308 and 243 cases. They likely make a gazillion of the base stock brass then divide it up between them, economy of scale. I suspect they make a lot more of them than the 6.5x47 also, there are a bunch of 243/260/308's out there and the 6.5x47 isn't chambered in any factory rifles, it's only in custom rifles which is a very small market.
I wouldn't look to find any used 6.5x47 brass, I doubt we'll ever see it in any significant numbers.