Mormon congregations are divided into wards, usually comprised of several hundred members. The leader of the ward is the Bishop. The Bishop helps ensure the ward members' physical and spiritual needs are taken care of. The Bishop spends many hours each week counseling individuals about their families, marriages, spiritual questions, assignments in the ward, relationships with other ward members, financial struggles, etc.
So how much money does a Mormon bishop earn? The answer is nothing. Even stake presidents, which oversee multiple wards, are not compensated for their time. Unless they're old enough to be retired, bishops and stake presidents work at full-time jobs to provide for their families. In fact, you might be working alongside a Mormon bishop at your day job without knowing it.
So why do bishops make the effort to serve? I would guess this scripture in the Book of Mormon reflects the attitude many bishops have about their church service:
"And now, if we do not receive anything for our labors in the church, what doth it profit us to labor in the church save it were to declare the truth, that we may have rejoicings in the joy of our brethren?" (Alma 30:34)
Because most bishops also hold a full-time job, their church service is done mostly on nights and weekends, supported by patient wives and children who sacrifice their husband and father so that he can help other ward members. This takes an extraordinary family effort, which is not unnoticed by the Lord. I've heard bishops' families talk about the divine help they've received in working together to help their husband or father fulfill his calling as a bishop, which usually lasts 3 -5 years. This article contains a particularly touching story about one bishop's wife realizing the importance of her sacrifice.
I have served as secretary to several Bishops and I can attest to the effort they devote to their callings. As secretary, my assignment was to help the Bishop organize his calendar; if someone needed to visit with the Bishop they called me and made an appointment. So I knew how much time the Bishop was devoting each week to visits. Or so I thought. Often the the Bishop would show up at our weekly planning sessions and start talking about additional visits he had made the past week that I knew nothing about. Some people don't want to call the secretary and they go to the Bishop directly, and often the Bishop was inspired to stop by and visit members on his own. That's the diligence offered by many bishops in the Church.
To learn more about how Mormon church assignments are made and carried out, see Serving in the Church at Mormon.org.