A "56k" dialup modem reaches speeds of about 52kbps (52,000 bits per second), which is a little shy of a 64k channel on a T1. There are 24 channels on a T1, and 28 T1s in a T3. Larger ISPs (ie not the ones run from a home garage) buy T3 and higher (OC3 up to OC12) for access to the global providers. These DS3 lines are expensive, about $4k-5k per month not including the internet access. OC3 lines hold about 3 T3s, and cost around $11k / month.
Now you can look at the numbers and think there is no way that 28 cable modem customers paying $40/month for approx T1 speeds on cable internet are covering the cost of a $5000 DS3 access line to the global provider. They're not. The ISPs are relying on the fact that internet data is very bursty. They can shove a few hundred customers on the same DS3 because each individual user is not using all 1.0-2.0Mbps 100% of the time. They're actually using it less than 5% of the time. If the network is slowing down around 4PM-8PM, primetime internet use for home, most people simply blow it off.
I think the cable companies should still be able to afford to give "unlimited" service for $40-50 per month for residential use. Once it becomes a business class service with guaranteed throughput, you will naturally expect the cost to be much higher.
The cost of BW is dropping rapidly as many companies are now offering cut-throat prices. An OC3 line 3 years ago would have cost 4-6x as much as today. Switches, routers, MUXes, even the fiber itself are all getting much, much cheaper.