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?