The priest in this story performs his clergical duties but he is certainly not wholeheartedly a "servant of God." Neither is he portrayed as being abjectly corrupt. He is simply a man tempted by the lust of money and the things it can buy (even if his thoughts turn to ecclesiastical needs!).
Note that Kino and Juanita had not been married "properly" by the church simply because they were too poor to do so. Evidently, the priest could have performed the ceremony as a favour to the poor couple but had either refused or not even thought about it. Upon Kino's discovery of the pearl, however, the priest now speculates on the big tithe Kino will eventually make and what all could be bought with it(more specifically, repairs which could be made on the church). He had not offered to help Kino and Juanita in their moment of need, but he is quick to remind Kino of his duty to give a substantial offering. Evidently, this priest does not display true interest in the lives of the people of the village but in the prestige, security and comfort his station in life procures.