" The main thing is to contain trading costs when the portfolio value is relatively small. Maybe $10 on a $500 trade is acceptable on a new small account, but when the account has grown in value it would be nice if the $10 cost is a smaller percentage of a much larger trade."
They are a higher cost than is implied in this statement. You need to look at the complete transaction. You bought shares and then sold them at some 30% profit and paid $20 for that profit. Roughly that is a profit of 0.3*500=$150 for which your commission costs are $20 or over 13%.
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