It's not. If any ebook had all the information you needed to make money online, the author would just use the techniques himself and scale them instead of putting them in a $7 ebook.
You're right. I believe the premise of the $7 ebook is that $7 is the "breaking point" price that people can buy something they're unsure about and not be upset that they've wasted too much money.
I get 50 of these "get rich" guides by e-mail every day.
If you ever need more "semi-reputable" information like that then just drop me note. For a small fee of only $7 dollars I'll send you a tarball of my spam folder.