Nothing! In most circumstances missionaries either pay for their own missions or receive help from their families. Those who desire to serve, but do not have the means to pay for their mission can receive financial help from other Church members who voluntarily contribute to a general missionary fund.
The fact that the missionaries do not get paid for what they do is a great testament to the fact that they feel strongly about the importance of sharing the Gospel of Jesus Christ. Generally speaking, young men serve for 2 years, and young women serve for 18-months. There are over 53,000 full-time missionaries currently serving in over 350 geographic locations throughout the world.
To learn more about missionaries click here.