The most useful missionaries to send are probably the ones with specialised skills that are difficult to find in the local church. But there are still substantial unmet needs in that category.
But to put finances in perspective,
Missionary Aviation Fellowship sends pilots to fly doctors, local pastors, and other missionaries in and out of remote areas in Africa. Necessary support is around $8,000 per month.
The average American missionary family needs between $4,000 and $5,000 per month. This would cover living expenses, housing, transport, health care, education of the children, and the resources needed to do the job (medicines for medical missionaries, computers for Bible translators, schoolbooks for teachers, etc.).