I'm not much of an ebay shopper/viewer, but my guess is the prices can fluctuate a lot depending on condition of the horns for sale. Any horn you buy on ebay is likely to require a sizable investment in repair work. So those prices don't necessarily reflect the overall trend. You might check prices on saxquest, vintagesax, and other reputable dealers to see what the horns go for in top playing condition. Finally, if you really do see a good bargain on a Big B, grab it.
I wish the prices really would drop, since I'm not in the business of reselling my horns to make money. I keep 'em and play 'em. But I doubt there will be any long-term downward trend on any of the well-know vintage horns.