Actually, "two bits" is twenty-five cents, not fifty. The American "dollar" derived from the silver Spanish "dollar", which was a silver coin, also called a "peso", that was worth eight reales (and thus the term "pieces-of-eight"). A "bit" was thus one real, or an eighth of a piece/peso, and a quarter-dollar was the equivalent of the Spanish coin that was worth two reales, or "two bits".
Something that is described as "two-bit" is being called cheap, petty, and of little importance.