The joke is that 599.99 is .01 less than 600. The difference is so minute that an ordinary person would normally overlook it.
However, CrazNoDale is implying that while Yoona is an ordinary person, he himself isn't ordinary at all. With this in mind, the audience and Yoona (who is at the receiving end of the joke) is to chuckle at CrazNoDale's attempt to denounce Yoona's accusation by using said difference.