Let me share some of my experience. I once posted a 3-month old a350 dual lens kit for a friend at BnS. It took a day and some heavy bargaining before I could let go at 37% lower than it's purchase price. While my 450D took only an hour to sell at 20% below it's purchase price. CANIKON resale prices are really higher based on the figures I see at the BnS.
But not so if you are comparing good (not G or high-end) Minolta lenses and Canon/Nikon mid-range lenses....