I agree, some lenses can be had for a good deal but I would call those mid-line lenses. For Canon I know the top level is the Red line. There is a 16-35 on Expedition that just sold for $1100 I believe.
If you want very good quality but don't want to break the bank check out the Sigma DX line. THey are very good and cost nearly half the price of Canon or Nikon Pro stuff. You may have to shoot a few before you find a real sharp copy but that is the same for the name brands. I have a 17-40 that I returned 3 times till I found the one I liked and this is not uncommon.
I think CL is the best place to find lenses for walk around and abusive situations AKA 4Wheeling. I don't bring the good stuff out because it will only fill with dust or worse. I use a Sigma 18-200 mm lens for Moab and other dusty environments. It does real well but not as nice as my Redline stuff.
Go check out some Siga stuff and read the reviews. You can usually get the same size lens from Nikon in a 4.0 for the price of a 2.8 in a Sigma with nearly the same quality glass. Check out B&H in NY they have the best selection hands down and great catalogs for you to drool on.