The image below (click to enlarge) shows an earning from our own Amazon Associates account for one random day (today, July 19th).
As you can see, it is only $12 – which won’t make anyone rich! – BUT, this is from ONE of our sites for sale (recipes/weight loss), which (like all our sites for sale) is not promoted at all yet. (This site also has several other affiliate programs – CPC, even easier than CPA i.e. Amazon – which have probably generated further income. Also notice that this particular site has 24-hours Amazon cookies, we have many with 90-days cookies – do the math yourself).
Anyway, this is just an example. Multiply this earning x30 days, add a few more sites to your portfolio, and spend a little time to create some traffic (which is not hard via social pages, seo etc.), and you will see how you can easily make a very decent monthly income with almost no investment (and very little work!).