Start off with an excel sheet. Each time you put in a trade, start off by imputing all the numbers in the excel sheet and testing your breakeven point under different conditions. That is the starting point. Do a sensitivity analysis of how a reduction of any of these factors can make a difference to your trading profits. That will put a lot of things in perspective. The bottom-line is that as a trader you must keep your visible and invisible costs under a tight leash! Non financial items like liquidity and spread costs cannot be quantified but you can make a provision for it. You at least now have a hang of your total costs.