A man and his wife decide to have sex

But the wife isn't comfortable with their son, Johnny, being able to barge in on them during the act.

The man has an idea.

He goes up to his son, "Johnny, let's make a deal. Stare out of the window and for every person dressed in black you see, I'll give you a dollar."

Johnny agrees and the man and his wife go off to quietly have sex.

A few minutes go by..
"Dad you owe me a dollar!"

Another few, "Dad, you owe me two!"

There is a period of silence from little Johnny until he exclaims, "Dad, a prostitute would have been cheaper! There is a whole funeral procession outside."