TI-83 graphing calculators.

Cost to make a TI-83 calculator: $15 to $20

Price you pay: $100 to $140

Texas Instruments, the manufacturer of the TI-83 calculator, has retained a monopoly in the field of American mathematics, allowing it to charge a lot for its products. Textbooks, like ones made by Pearson, often feature illustrations and exercises that pair with TI-83s, so students are forced to buy them.