While these appear to be mostly guesses, there is an idea that Starfield will generate somewhere between 3-5 million Game Pass subs by itself. 5 million subs would be 20% of all Xbox Game Pass subs to date, as the last reported figure was 25 million in January of 2022.
I bought the Starfield upgrade for $30 instead of that huge $90 bill. I’ve been up since May, kinda sucks going month-to-month at the moment