Last week a friend asked me which dSLR she should buy, and I recommended the Canon EOS 400D -- partly because that's my stock answer right now, and partly because she used to have one of Canon's film SLRs. I don't have strong feelings on Canon versus Nikon, but it makes sense to stick with what you know.
I also suggested that she might want a better lens than the EF-S 18-55mm lens normally sold with the 400D, and recommended the EF-S 17-85mm. It's a good general-purpose zoom lens, with image stabilisation for taking photos in low light. What's not to like? Well, she was horrified to discover that it costs £500 (£400-ish if you shop around).
I'm not going to suggest that £500 is the sort of money that you find down the back of the sofa. Well, not down the back of my sofa, anyway. Nor am I going to argue that it's good value, because that's a hard call to make. However, her reaction did get me thinking about the cost of lenses.
I wasn't surprised by the price, because Canon's zoom lenses start at around £200. Getting the wide end out to 17mm -- the equivalent of 28mm on a film camera -- adds to the cost, and so does the image stabiliser. In other words, I expected it to come in at £400-500, which is about the same as the body-only price for the 400D (again, if you shop around, you can get the two together for £800-ish).
If you regard the lens as an accessory for the camera, this probably seems outrageous, but if you take care of it, a decent lens will almost certainly outlast the camera body -- especially at the moment, when sensors and other camera technologies are evolving much faster than lenses. Beyond that, the lens is a key element in the production of your picture -- if it can't produce a sharp image, increasing the number of pixels on your sensor will just magnify the defects.
I can't easily justify spending £400 on a lens, but then I can't easily justify spending £400 on a camera -- or any other gadget. If, however, you're upgrading to an SLR to improve the quality of your photographs, it's worth thinking about both parts of the equation.