According to the revealed scriptures, if a spiritual master can convert even one soul into a perfectly pure devotee, his mission in life is fulfilled. Srila Bhaktisiddhanta Sarasvati Thakura always used to say, “Even at the expense of all the properties, temples and mathas that I have, if I could convert even one person into a pure devotee, my mission would be fulfilled.” It is very difficult, however, to understand the science of Krsna, what to speak of developing love of Godhead. Therefore if by the grace of Lord Caitanya and the spiritual master a disciple attains the standard of pure devotional service, the spiritual master is very happy. The spiritual master is not actually happy if the disciple brings him money, but when he sees that a disciple is following the regulative principles and advancing in spiritual life, he is very glad and feels obliged to such an advanced disciple.
From Srila Prabhupada’s purport to Cc Adi 7.91