I can see how someone might make 5,000,000 a month -- but that would be gross... the costs of getting people to his site, plus the affiliate costs would drive that down tremendously. In my personal experience, the CPM ad-revenue I get from google is far outweighed by the costs of advertising with them.
For my lowly site, I pay up to $10 a day in adwords advertising (that equates to 1 book, so any sale in a day and I break even for advertising costs).
In that same time period, I make anywhere from 10 cents to 2 dollars a day in adsense revenue. This makes sense to me.. My site exists to advertise my book and nothing else. There's like... 3 pages "Hi this is the book" "hi this is the payment page" "hi this is the thank you for buying page" -- so I'm not going to get a lot of page hits per visitor.
With another site, a person could possibly have many, many pages of limited content, all designed to hand out morsels of information, while at the same time pimping the book... I tried this for a bit with a blog, and simply decided the work didn't justify the additional pennies per visitor extra I'd make.
I have very few other costs than that advertising (which can never exceed $300 a month or so). Paypal charges me .50c for the transaction. My webserver costs $50 a month.
So I'm just an author in a very tiny niche market, with a small book that sells for $10 a pop. I sell about 3-10 books a day (more on weekends usually, less in the week) for a good average of 100-150 a month. (In October, my current sales are 37 purchases, 2 returns for 35 units total -- not a bad week).
That's $1500 average (September was a recordsetting month, 200 books sold, for $2000 gross).
Subtract the $300 in advertising, the $50 website costs, and $75 in paypal transaction costs, and I'm making a little more than $1000 a month -- with just one little book, in one tiny niche market. -- If an author found a larger market, and could promote it inexpensively (that's where the affiliate marketing comes in handy), it would not be surprising to me to see that author pull 5K, 10K, or even more a month.
-- Now, what if that author has multiple books? In the same market would be preferred... because his per-book marketing costs go down -- and his adsense revenue goes up (as people have to hit multiple pages to learn about each of the books).
So, having done it, on a tiny microcosm, I can see how others could do this too, on a much grander scale.
Is it possible someone's making 5-million a month? Yeah, it is. How many people could do this? That's hard to tell, but the correct answer is somewhere near "not very many." However I do not think it is out of the realm of possibility for anyone to be able to pull $5K a month or more self-publishing ebooks online if they wanted to devote some time to doing it.