Are we talking about ECN fees? They are separate from comission that brokers take. You can get them even in DAS simulator. Go to top bar "Trade" -> "Account Report". I'm in simulator and calculate costs 0,0035 per share + ECN fees shown in Account Report. The more you have tickets/scale, the more they add up.
Then there is different lot's. "Board Lot" is block of shares that are measured in hundreds(100, 200, 300, 1000 etc.). Block less then a hundred(60,70, etc) is "Odd Lot" and numbers between hundreds are "mixed Lot". All these have affect ECN fees. It's quite easy to understand. If you add liquidity like Board Lots you get rewarded by lower fees. If you make it harder to the market to fill orders you have to pay like in Mixed Lot.
My biggest weakness is overtrading and these fees has major impact if you overtrade. And Roberts post shows the range were i have seen them vary. But it's great that DAS calculates them so you don't need to guess or average them.
In few strategies i have tested by using LIMIT orders because it adds liquidity and it not only reduces ECN fees but you can get rebate for adding liquidity, hence reducing fees even more. Never seen these rebates in live or done research so not sure does it really work in practice. But at least ECN fees are smaller. Mostly i just try to scale fewer times than Andrew because it'll eat small account if you scale six times on small share sizes.
I still wouldn't start using LIMIT orders in fear of fees. Like Andrew says, at the beginning(or for a while after) it's not important at all. You need to learn everything else first and then you can make adjustments so you can be more cost efficient. But it is still good to know these things exists.