I dont know why they are so cheap. I have bought slides that look the same from Lee Valley and the difference as far as I can tell is that the BCW ones dont come with instructions or screws (I didnt use the screws that came with the Lee Valley ones anyways as they were either too long or too short).
Finding them at BCW made my original huge pull out drawer project possible in my own kitchen. I even removed the kick plate from the island and installed a huge drawer just for flat oven pans and muffin tins.