I don't want to argue with you, but you are misunderstanding the intention of my calculations. If you want to know how long it takes to profit, over a length of time, you can use Hellcom's calculator. My version is to tell you how long it's going to take to pay for that new bonus. The calculation is not wrong, or misleading. If you are buying a new piece of gear, that means that you have used x amount of hours... you have already done the hours put in. This plugin will tell you how much, exactly, you lost.
Say you got 250 GF and you make 500k per hour. You find some new bracers, with the exact same stats, but 5 GF more then your old ones, for 500k. Your new GPH after the upgrade, will be 507k per hour, the increase will be 7k per hour.
Now, the first 500k, that's money you would have made anyhow, regardless of whether you bought the new bracers or not. That money is already yours.
But the 7k / hour, that's extra money. But investing in the new bracers won't actually pay off until these 7k / hour have turned in to 500k total. Which will take a total of around ~70 hours. In other words, for the next 70 hours, you would've made more money by not upgrading then you do by getting the upgrade. And another 70 hours after that, you will have made 500k extra because of that upgrade.
70 hours, that's not very long, it's 3 days of botting before you turn a profit on your new investment.
But what if the item had cost 1,5 millions?
In that case, it would've taken you 210 hours before it paid off. In other words for the next 210 hours, you would have made more by not upgrading then what you do by upgrading. That's almost 9 days of botting before the item pays off, in that case, is it worth making the upgrade? Or should you just keep your current item and maybe look for something else?
If the item costs 3 millions? Wow, then it will take between 17 and 18 days for it to pay off, that's really not worth it at all.
This, is the reason you use the calculator.