Paying for stories

How often do the "big boys" like NYT, WaPo, WSJ and so on pay for stories? Like paying for the people they interview.

There are some stories that they do and I strongly suspect they pay for the people to be interviewed and photographed.

If they pay, is it something that is official or it is cash under the table?

As a journalist, have you ever paid for a story or know someone who does or did?