No, Nikon lenses are not twice the cost of Canon lenses whether you are comparing euivalent products from Canon or Nikon or whether you are comparing the same product in different mounts from an independent lens maker.
See http://www.bhphotovideo.com/c/search?Ntt=sigma+70-300&N=0&InitialSearch=yes&sts=ma
- Sigma 70-300mm f/4-5.6 APO DG Macro Autofocus Lens for Nikon AF-D $199
- Sigma 70-300mm f/4-5.6 APO-M DG Macro Lens For Canon EOS Cameras $199
Or roughy equivalent products from the primary vendors
- Nikon USA AF-S VR Zoom-NIKKOR 70-300mm f/4.5-5.6G IF-ED $586.95
- Canon USA EF 70-300mm f/4-5.6 IS USM Lens $549.00
All Nikon "AF" lenses are autofocus. All Nikon "AF-S" lenses have the focus motor built into the lens. The majority of Nikon lenses are "AF-S". Nikon do still make a few manual focus lenses and a few AF-D lenses whose autofocus depends on the body having a focus motor. Canon also still make manual focus lenses (but I'd guess fewer).