Supply and demand. There are so many acting schools for adults that to stay competitive, the market price stays relatively low. The more reputable classes tend to be slightly pricier.
For kids, however, there aren't as many schools. Plus, because parents are paying, the schools figure parents are willing to spend a lot of money on their little darlings.
For more tips and articles by top entertainment industry coach, The Greenlight Coach, visit www.theGreenlightCoachBlog.com