Most of adsense millionaires used this technique: they select most paying keywords (there are several paid tools for that) and they make microsites about this keywords. With some black-hat SEO, they make fortune with adsense. They write codes, and automate the whole process. Some make content aggregation, they don't write anything, everything is automated.
Google updates its algorithm to prevent this kind of abuses, but still, there are too many people smarter than google engineers.
I've read too much about this kind of business. 7 years ago I made a small site to test if it really works. I made a blogspot site with only 2 articles, it brought me 100-200$ per month. I was in high school that time. Than I read some interviews from big content website owners that they make too much money with their website. And I made a decision that I'll learn SEO and build a big content website with a unique idea. I was 5 years long a premium user in SEOmoz. From articles by Rand Fish I discovered Hacker news. And the community blew me away.
I started university and started to build my "unique idea content website" but it was never finished because of Hacker News. I saw that although it's still a good idea to make money with a content website, there is better way to make money and it's more passive than that. It's just simple idea. Start a startup.
I'm still a student at a medical faculty. Last year I started a startup to see what it looks like and if I can build something useful. I bought a domain name for 10$ and 9$ hosting from hostgator. Now it makes 20-30k $ per month and I don't do anything. I'm the owner of my small company and I have 12 workers and one CEO to organize the work, and I continue my study at the university.
After seeing how it's simple to start a startup, (of course starting is not so easy and the idea, marketing also important) I think I'll continue my life as an entrepreneur, not as a doctor.
People fail because they ignore the need to create value. They fall for sucker schemes that promise vast reservoirs of cash on tap if only you know the secret. There are rare opportunities that arise from market inefficiencies, but you can bet your ass that if someone has identified one of these opportunities, they're not going to cut you in on it.
If you want passive income, you need to create something of real value, then work out how to sell it. It's usually easier to identify the market first, then find a product to sell to it, rather than vice-versa. There are an infinite number of possible products, but relatively few viable markets.
Can you give a few examples that are doable on a small budget online?
The thing is, it's generally easiest to create lots of value (and correspondingly, earn a lot of money) in an area where you have very little competition. 10 years ago nobody understood the internet, so passive income online was easy. 5 years later it was doable, but harder. Today you'll probably get a much higher RoI by learning R and doing some Data Science consulting.
If you can put some graphs, charts ,or infographic in it to make it more insightful.
Here is great example : http://www.marketinghotsheet.com/
One of my friend used to do it, he had made good mullah through it.
PS: What might work for someone maynot work for you. You have to find that unique thing (idea) that might work for your case. Based on your skill set.
If you think about the end result (pay rent in your case) then you maynot be able to succeed! Okay I don't want to talk philosophy here buy you got the point, didn't you?