I'd be guessing that it would approach $2,000 upon release (ie approx. 50% increase). But they'd probably add IS and beef up weather sealing. A new 100-400 could be around $2,500. As a "cheap" lens person, the current 400/5.6 was a simple decision for me - Noticeable cheaper than the 100-400 and arguably faster focusing and sharper at 400/5.6. But if the price gap percentage between the two narrows, I might be tempted by a new 100-400. A tough decision to make when you are weighing up two hypothetical lenses.