Used prices fluctuate. In general most second hand lenses are cheaper now than they were a few years ago, but then again the whole world has been feeling a little poorer than it was a few years ago. If you are buying lenses in the hope it's a good investment, well, you're probably going to lose money. This isn't a great time to sell lenses to be honest.
On the flip side of the coin, I just bought a da12-24, fa28, fa43, dfa50, da70, dfa100, fa135, and a da50-135 in a bargain basement job lot (all in perfect condition). I won't tell you how much I paid, because everyone will hate me!