Trading is very difficult. Not only is it difficult to find profitable strategies, you also have to overcome slippage and commission costs. Slippage is difficult to estimate. Commissions costs, however, is easy to calculate. Personally I pay .001 per share in clearing plus ECN rebates and taxes/fees.
So far in 2012 I have paid 6725 USD in clearing and fees. That equals about.0028 per share (trading is very slow at the moment so not many shares traded this year). The reason I pay so much (considering my clearing fee is .001) is because I remove a lot of liquidity. SEC fees etc. also add up.
In addition I have paid about 250 USD a month for quotes.
This illustrates how much money one has to make just to break even to keep up with the costs! And this is just a fraction of commission costs compared to 2008 where I traded many times more.