Math Zoom has been conducting math summer camps for some time. Now they have started the Areteem Institute, "a college preparatory school for the gifted", which offers online classes in math, physics, and English
http://areteem.org/en/online-courses/course-schedule . They also offer in-person classes in Irvine, CA. They say this about costs:
The tuition for a full time enrollment is $16800, which covers three regular terms and six weeks summer residential intensive. The tuition for a single course enrollment varies from $350 to $850 depending on level of the course, the length and how many times of the classes per week.
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Areteem has very limited need based financial aid available for qualified applicants. There are third party foundations and organizations that provide scholarship.
My eldest son is taking an AOPS course and doing ALEKS and IDEA math, so we we are not looking for more courses right now. I will be interested in the experience of anyone who takes their online courses. An established online school for the gifted is Stanford University Online High School (grades 7-12)
https://ohs.stanford.edu/ .