So, I was scrolling through Reddit (mod edit: please do not link to competing forums or groups). This guy found an untapped niche and then invested $2k in content to build a $1k/month income stream, that's valued at at least $35k if he wants to sell the site.

Here's the process to find niches like that:
Log into Ahrefs and navigate to the content explorer.
Chuck a very broad generic keyword into the search bar. If you're looking to build an affiliate site then something like "best" is a good choice.
Change language to "English"
Turn "Filter explicit search results" on
Then press "add filter" and select "domain rating", leave the from box empty and put a low number (around 10) in the to box.
Then add a "page traffic" filter, go for a fairly high number up to 10,000.
You will now see a list of pages that get more than 10,000 sessions a month on sites with a domain rating of less than 10.

While not all of these articles are something you'd want to build a site around you are sure to find a few diamonds in there which prove you can build high traffic sites without having to spend countless hours and $$$s building links.

This process found him two massive (40k+/month) keywords in the same niche which were very low competition.

He also made a video on how he approached the project:https://www.youtube.com/watch?v=IHB6g7S_4sA