The most sane pricing would probably be per bit with surge pricing during congestion periods. But customers would flip out at that.
It's a measure to prevent customers from watching video on-line and stopping their TV service.
I already pay ~$1500/m/rack in bandwidth (5Gbps, 95th percentile, mixed transit product + interconnect fees for an exchange). I'll happily pay $100/m for 100mbit 95th percentile bw at home, with the same throughput guarantees. .
9am-6pm Mon-Friday is charged at 1.25-10x (ratio depending on the technology/broadband type) the price of bandwidth outside of this peak time.
Granted, AAISP customers tend to be more technical and are willing to pay more for an clueful ISP.