The actual bandwidth used by a 300M customer and a 1000M customer is usually the same. The provider's upstream contracts would not change much if they put every one of their customers on 1000M.

But their pricing is regulated. The arm and a leg they charge for 1000M service is largely a function of that regulation, even though it hardly affects their costs otherwise. You might think that if the infrastructure costs are the same for 1M or 1000M and that the infrastructure costs are 90% of all costs, 1M and 1000M could be priced within 25% of each other. It might seem that a price jump from $100/50M to $1000/1000M is unreasonable. But the U.S. government right now is handing out $billions to rural providers to build out fiber, then requiring them to charge ridiculously high prices.

This is asymmetric with no disclaimer on their advertising page from an entrenched local monopoly in the north east (a small one)