There are literally millions of micro-niches online where spending x will yield x per month, fairly consistenly. Just find these, and scale.

For example: Udemy courses. I have one that does $3,500 a month on the high end, and about $500 a month on the low end. I know a guy who gets about the same average but has created roughly 40 courses. He, as you can imagine, makes a little more than $5k a month.

Btw, my investment in the course was no more than $200 total. Plus of course about 20 hours of my time.