The 40,000 spent was a large sum of money in 1975. You could buy 6-7 automobiles for that. Takes money to make money.
Also in the letter note name : micro-soft and the Albuquerque mailing address.
I Couldn’t really embiggen the letter in the article at least on my phone. but here is a version I found
https://lettersofnote.com/2009/10/08/most-of-you-steal-your-.../
"In 2008, Homebrew member Lee Felsenstein recalled similar doubts about Gates' $40,000 number: "Well, we all knew [that] the evaluation of computer time was the ultimate in funny money. You never pay that much for the computer time and I think that research will show that they were using someone else's computer time; someone else was paying for that. It could have been Honeywell where Paul Allen was working. So we all knew this to be a spurious argument."
https://en.wikipedia.org/wiki/An_Open_Letter_to_Hobbyists