Depends on the lens, really. The higher quality lenses keep their value pretty good. Some get more expensive, especially if they go out of production. An example is Canon's 50mm f/1.0L (yes! f/1.0!). It's selling now for double what it retailed for. I saw one a window about a month ago for ÂŁ3200. Thought it was the 85mm f/1.2 first, because it's so big, but then I noticed the price. I thought no one would ever buy that, but two weeks later, it was gone!
Something else that's happening now too is that the Yen is getting so much stronger, so lens prices (for Canon and Nikon, at least) are going up. The 24-105L I had, I bought for ÂŁ700, but they're retailing for ÂŁ900+ now, and I see second-hand ones going for ÂŁ800. Not a bad return on investment...