I don't have anything wrong. I am talking about gross margin. Gross margin and net profit are not the same thing. You could say gross profit and gross margin are the same thing, but gross profit is not really terminology people normally use. Net Profit is after you factor out all your expenses.
If I buy a can of corn for $.50, sell it for $.75 cents my gross margin is 50%. My profit is going to be determined by how much I make after I factor out all of my expenses. That means all my payroll, overhead and other expenses. Perhaps I only make 2 cents after all my expenses. My profit would then be 4%. That does not change the fact my gross margin is 50%.
You have to actually factor in all your costs to determine your proiftability. When you are talking about Arbitrage this includes your hosting bills, any costs associated with setting up your sites, software/hardware deprectiation etc, as well as your time for actually managing and setting up the campaign. Only after you factor in those numbers do you come to an actual net profit number.
By the way you could call gross margin, Return on Investment, except I consider all my related expenses to be part of my investment, so I would never track it that way. People need to account for their time and expenses when doing these things.