A noble rank worth twenty thousand was too tempting to resist.
Even though Black and White Xuanjian was the leader of the State of Qin's Zhao Wang, the organization ultimately operated in the shadows, unsuited for the light of day. Noble ranks, on the other hand, were severely limited in number within any state, and very few ever managed to attain one. Most importantly, noble ranks were hereditary, carrying privileges and lands that could be passed down through generations.
If the State of Qin had allowed people to purchase noble ranks with money, Black and White Xuanjian would have sold everything he owned to scrape one together. As for the Qi State, however, he decided against it after a moment's thought—it wouldn't survive much longer anyway.
Chapter 726