Markets are dynamic and change all the time.
Used lenses are worth a lot more today than they were a year ago. This is due to the global recession and currency weakness, as a result of which Nikon and others have put up their prices. How much they will have gone up will depend upon the exchange rate between Japan and your country.
Outside of this unusual situation, the longer term trend is for popular lenses to fall in price by around 20% per annum. This compares to the bodies falling by around 30% per annum.
I see no significant falls due to people switching to FF. The FF market is still quite small, whereas many more people buy the cheaper D40 type cameras, then want to upgrade to something better than the 18-55mm.
I suspect that the best bargains remain the kit lenses, which get dumped as users upgrade. I reckon the best bargains among them are: 18-70mm, 55-200mm, 50mm f1.8 AF-D, 24-120mm VR. Provided that you buy carefully, used lenses are often great value for money.
S.
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Wait, watch, listen, then pounce !