It depends on how many people are in your family. Last time I looked into it, for 2-3 kids in a family I believe it was aound $40,000 but I could be wrong. It was atleast in the neighborhood of that amount, though. When I inquired to a teacher, she said that the families are notified if they qualify- maybe because they might be involved in other govt programs.
A good low cost alternative is Pep 'Pilot'. Your child can be a community typical peer student and pay a low tuition for age 3 & 4 preschool years at select schools. Admission given to local kids first, then open to kids out of boundary. Inquire at specific schools for availability and application info.
http://www.montgomeryschoolsmd.org/curriculum/pep/locations.shtm We had a great experience doing pep pilot, and know several other happy families as well.