I've been testing out various cheap Canon lenses as well as my normal Canon L series that I normally use for studio shoots. If you are looking for a cheap lens to shoot studio portraits with then you can't get much better value than the Canon 35-80 lens that was sold with many cameras such as the Canon 500N during the 1990s. For portrait photography the Canon 35-80 is far better focal length than the Canon 18-55 lens that comes supplied with the camera. The Canon 18-55IS lens is too wide for flattering portraiture so the 35-80 range (equivalent to approx 50-120 on the Canon crop sensor) is a much more suitable focal length.
I picked up an example of the Canon 35-80 lens on Ebay recently for £22 and tested it yesterday on a shoot using my Canon 550D camera, which has the same 18Mp output as a Canon 600D.
The results in a controlled studio environment were very good although clearly the advantages of the L series pro lenses are far more than the improved optical and build quality. You also get the f2.8 aperture as well as the image stabilisation.
The shot above was taken with the Canon 550D and Canon 35-80 lens that is an amazing value portrait lens for studio photography.
Studio hire, £30 per hour, lens £22.
There can't be many lenses the cost less than the hire of the studio!