You'll find a few good deals from the dealers, who obviously won't want to be stuck with older models. You may also get good deals from larger dealers who will routinely buy up all of a manufacturer's stock of a soon to be discontinued model at bargain bulk prices. They'll then pass those discounts on - well, usually!
I'd usually say that with the launch of every new model, the value of used models drops a little. However, when I've been looking around for some good bargains in the PSR-S9xx series recently, I've been surprised to see just how well the used prices for private sales have been holding up - or even increasing. That's for the UK, I can't speak for other countries.
And the PSR-SX is still speculation, as far as I can tell.