"I published a series of eight-page full-color, folded posters (with headlines and articles printed on the back) and charged the same cover price as magazines with ten times that number of pages. The result was dubbed a 'poster-magazine' and sold millions of copies around the world. Nobody had told me it couldn't be done because nobody had done it before."
The pattern I see here is: He saw a way to take an existing product which normally had a short lifetime-value to its purchaser, make a slight (and inexpensive) modification to it, and thereby extend its lifetime-value to the purchaser, such that he was able to simultaneously cut other features from the product (and thereby drastically reduce the cost of producing the product) while also maintaining a price-point that customers had become accustomed to paying, and thus realize a large profit.
'Passive income' ideas
Thoughts
'Passive' seems to imply:
you have your desired amount of money coming in without you needing to do anything
you don't need to spend a lot of time continuing develop the product to fend off competitors
This in turn seems to imply that either:
you're in an uncontested market
a niche market, like Kalzumeus' bingo card creator
a natural monopoly (network effects)
you're in a contested market, but the switching costs for your customers are high enough that any new development in a competing product would not be enough to make it worth it for your existing customers to switch, and you don't care to competitors take new customers (or new customers don't pop up that often).
I remember reading about an insurance product that had this characteristic. I think it was in NJ.
So, with all of that in mind, it seems like what your best bet is as a little guy is to go after some super-niche and 'hidden' market, like the bingo-card-creator.