The problem is that most of the costs of providing broadband are fixed costs. To run a coaxial cable from their headend to your house, and to provide employees and crews to repair it and install it all costs the same whether you buy 10 megabits or 2 gigabits
The variable costs--things like backbone bandwidth, paying for transit, doing node splits, etc are a smaller part of the overall cost than you might expect
This lower end of the market, though, is being taken up by Fixed Wireless (T-Mobile and Verizon primarily) for example you can get Verizon home wireless internet for as low as $25. My mom has it, and it's decent 50 megabit service for a low price