A “throw” in this context refers to having sex with a prostitute for a short while.
Holden decides to return to the hotel. In the elevator, Holden feels very depressed. This is how he gets himself in a “big mess.” A man asks him if he is interested in “tail,” meaning a girl. He says that he is twenty-two when asked. His judgment is questionable and he is aware of that. He just wants to make himself feel better.
"Uh huh. Well, how 'bout it? Y'innarested? Five bucks a throw. Fifteen bucks the whole night." He looked at his wrist watch. "Till noon. Five bucks a throw, fifteen bucks till noon." (ch 13)
Holden is actually not interested in a prostitute. He is paralyzed by depression. He lets the girl come to his room, but is not able to have sex with her. He just wants to talk. The girl gets annoyed and asks for more money, but he does not pay.
The incident with the prostitute ironically demonstrates the lack of courage Holden was complaining about at the beginning of the chapter. He does not have the courage to go to the bar and get drunk, and also can’t tell the elevator operator he is not interested in a prostitute. He thus feels even worst about himself, because he does something against his moral code. He is in danger of becoming what he hates: phony.