In "The Catcher in the Rye," what exactly does Holden mean by "just a throw"? when he is conversing with the elevator guy about the prostitude

The irony of the moment is that Holden is taking a moral stand in the midst of a highly immoral moment. A "throw" refers to sexual intercourse. He spends the novel fretting over a possible sexual tryst involving Jane Gallagher. Yet he has no qualms about entering into this exchange.

Holden raises a fuss over the unfairness of the exchange between himself and a pimp. While spending most of the novel as a defender of the innocence of young boys and girls, he gets upset about the change in price. By defiantly arguing the price of the agreement, he is in fact calling her a five dollar whore, not a ten dollar one.