1 According to MormonMissionPrep.com, the mission cost is around $400 per month, or about $7,200 for an 18-month journey. 2  Laytreasuresinheaven.com broke down what the mission expenses would look like if the funds weren’t distributed equally.hey are reported to have more than 80,000 full-time missionaries worldwide as well as over 22,000 part time church service missionaries. Most full-time LDS missionaries are single young people in their early twenties and late teens, as well as older couples who no longer have children in their homes.