6 series prices

[Speed addicted]

I've been keeping an eye on 645ci prices since the car was launched, they seem to be zooming downwards at quite an impressive rate.
The cheapest manual I've seen so far is £24K, and £18.5K for an auto 630, any idea why they're dropping so quickly?
And any thoughts when the depreciation tends to level off on cars like these? I'd probably be looking to get one in about a year, hopefully for mid to high teens, just wondering if this is feesable?