In the real world, you are worth 500K. You go to a stockbroker and buy a security worth 100K. He charges you a fee of 1K. You are now wortk 499K because you paid the fee.
Similarly, if your net worth at HSX is whatever and you sell the security, the value of the sold shares is added to your cash, so the net worth would stay the same. But the commission is subtracted from your net worth,
As to the plaint of my friend ss, 100K shares doubles the profits from 50K (or losses ) and the commission is doubled. It is the same as if you bought or sold 2 stocks of equal value at 50K. It is just that any specific amount gained (ie 50K) on a specific security is half the percentage yeild. Put another way, when 100K gained might have looked good on 50K shares (2 per share) 100K gained is only 1 per share at 100K shares.
Whether it is 10K, 20K, 50K, 75K or 100K or any other share amount, if the rise on a long position or the drop on short, is less than 2pct, you have not made enought to cover the back and forth commission. HOWEVER, part of the strategy sometimes is to cut your losses, pay the commission and go the other way. How many people would have loved to short GM or Enron when they were tanking, even with brokerage commission?