For some the camera is but an accessory to the lens and for analog / chemical photography the film and the lens are really the two things that make or break your image. With digital cameras however the sensor takes the role of the film and also the chip processing the data is very important for the quality of the image. But nevertheless the light has to travel through the lens first and even the best camera will not be able to fully recover what was lost or mingled with by a bad lens.
There are lenses for $ 150 or less but you can also spend several thousands for a lens. This large variation in lens pricing is mainly due to the different features and quality levels lenses offer and not so much due to what influences price heavily in other consumer electronics products - design, popularity or brandname. Nonetheless you'll have to spend a couple of extra bucks if you buy an "original" Canon or Nikon lens. But while the Canon EF 600mm f/4L IS II USM which costs more than $ 10,000 is certainly one of the greatest lenses ever build it is not the right lens for you while you are hiking on the Appalachian Trail (unless you enjoy the 9 lb additional weight in your backpack).
The following lenses have already been tested and their reviews will be published on LENSTESTS soon:
Canon EF-S 10-22mm f/3.5-4.5 USM
Canon EF 16-35mm f/2.8L II USM
Canon EF 17-40mm f/4L USM
Canon EF-S 18-135mm f/3.5-5.6 IS
Canon EF 28mm f/1.8 USM
Canon EF-S 60mm f/2.8 Macro USM
Canon EF 70-200mm f/2.8L IS II USM
Canon EF 85mm f/1.8 USM
Canon EF 100mm f/2.8L Macro IS USM
Canon EF 100mm f/2.8 Macro USM
Canon EF 100-400mm f/4.5-5.6L IS USM
Note that the lab tests for these lenses are already done and the image quality test shots are available in the comparison test boxes. If you don't like to wait for the review to be published go ahead and check out the IQ test shots right now: click on one of the above lens thumbnails to get to a completed review. Then, go to page 2 of that review to find the IQ test boxes. Click on "Choose lens to compare with..." to load the test shots for any lens that already went through the lab test.