https://www.quantopian.com/help
Search page for cost_basis. This is copied from there.

def check_positions_for_loss_or_profit(context, data):  
    # Sell our positions on longs/shorts for profit or loss  
    for security in context.portfolio.positions:  
        is_stock_held = context.stocks_held.get(security) >= 0  
        if data.can_trade(security) and is_stock_held and not get_open_orders(security):  
            current_position = context.portfolio.positions[security].amount  
            cost_basis = context.portfolio.positions[security].cost_basis  
            price = data.current(security, 'price')  
            # On Long & Profit  
            if price >= cost_basis * 1.10 and current_position > 0:  
                order_target_percent(security, 0)  
                log.info( str(security) + ' Sold Long for Profit')  
                del context.stocks_held[security]  
            # On Short & Profit  
            if price <= cost_basis* 0.90 and current_position < 0:  
                order_target_percent(security, 0)  
                log.info( str(security) + ' Sold Short for Profit')  
                del context.stocks_held[security]  
            # On Long & Loss  
            if price <= cost_basis * 0.90 and current_position > 0:  
                order_target_percent(security, 0)  
                log.info( str(security) + ' Sold Long for Loss')  
                del context.stocks_held[security]  
            # On Short & Loss  
            if price >= cost_basis * 1.10 and current_position < 0:  
                order_target_percent(security, 0)  
                log.info( str(security) + ' Sold Short for Loss')  
                del context.stocks_held[security]