You get 10TB of bandwidth for a dollar. Once average usage per subscriber goes over that, then we can talk.
Assigning a single rate ($/TB) requires you to make some assumptions that are at risk of being violated in exceptional circumstances. Using variable pricing to charge more during peak hours is too complicated for consumers to keep track of and their options for changing behavior are limited, so this earns the ISP more money but doesn't eliminate congestion during peak hours.
Why? It works for electricity.
The cost curve for internet service during peak hours is a lot steeper. I think it would take much more than a 3x price multiplier during peak hours to get any noticeable demand reduction beyond what streaming applications already do by dropping down to lower resolutions automatically. (Assuming that the base cost for off-peak usage is remotely realistic, ie. orders of magnitude lower than the metered prices we pay for cellular data in the US.)