I would think that if someone manufactured a similar product which costs $15, the TI brand name wouldn't compensate for the price difference.

Perhaps it's a complex product and teachers won't switch products because they've invested so much into learning how to use the TI? And teachers decide what calculators are required for a given class? Maybe a competitor would have to get to the teachers first and convince them to use a cheaper alternative.

Another possibility: perhaps TI has some vital patents?

I know many of my classes required that I own and use a TI throughout high school. Other types of calculators where not allowed.

It might be worth paying some extra for TI because the UI is consistent with what your peers, textbooks and teacher uses.