Watching the order books on BFX today, the spread was pretty awful for hours. It would have cost you $2.70 from the highest bid (not including exchange fees) to buy 1btc. About $1.70 spread and another buck to gather 1btc from small bids.
What is the strategy if you are doing 100btc or 500btc trades? In most cases, you are going to lose $2-10 on slippage. Just sell a few coins at a time as you get close to your exit? Or only do large trades on big movement?