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.
When Maurice makes the offer to Holden of "five bucks a throw," he means the fee for having sex with the prostitute is five dollars. If Holden wants to spend the entire night with her, the cost will be fifteen dollars. Holden, who is depressed, takes up the offer without thinking clearly, he says, and regrets going against his principles. His encounter later with Sunny and Maurice in his hotel room proves that his regret was well-founded. He was sorry he'd let "the thing start rolling." Holden was embarrassed to admit he had no experience in sex; later he must make up excuses to Sunny.