The fee for each AP Exam is $91 (2014) and students have the option of taking College Board (American not-for-profit organization) approved courses and then taking the College Board's examination in the Spring.
As of the 2015 testing season, exams cost $91 each, though the cost may be subsidized by local or state programs. Many high schools in the United States offer AP courses, though the College Board allows any student to take any examination regardless of participation in its respective course. Therefore, home-schooled students and students from schools that do not offer AP courses have an equal opportunity to take AP exams. 
