think of negative costs as the benefit you have when waiting to trade at a more advantageous price.
For instance, the current price when you decide to buy a stock is 50 (which is also your benchmark). You take a limit order position to buy the stock at 49, which is below the market price. If the price falls to your limit order, then you would gain a negative cost(benefit) from buying the stock at 49 instead of 50.