what kind of data the organizations are buying?
Any possible data they can gather.
Mostly buying habits you possibly have.Websites you visit. The time you spend on a website. The amount of times you click on a specific AD.
But also metadata like IP's, your OS (desktop / mobile), possibly GPS..
what is the use of that information?
Simple. Advertising.
They want to show you an AD which animates you to buy something.
A few months ago i have read an article about an AI in combination with big data.
They are trying to build a system which does show AD's on a users mobile of something the user wants, which is available in a shop just a few minutes away.
if someone buy my personal data, how many that person paid for it?
That differs a lot.
There are packages available with millions of user data (e.g. fbook). These will be relatively 'cheap' on a 'per person' rate.
But one could also hire a private detective to collect data.
This really depends on the method used to gather those information.
and thhe most important, how i can protect my data from those companies?
Don't give it away.
You should stop using social networks or applications which do gather and distribute your personal data.
Additionally you could use something which blocks javascripts on a site by default. (e.g. NoScript browser extension).
You will be shocked how much scripts are running on a website just displaying 'news'.
Those are mostly tracking services. The whole purpose is to track users across sites to create a profile.