10Mb CDR on 100Mb 95th burst transit typically less than £/$/EUR300 install and less than £/$/EUR100 per month total (less than 10 per Mb) for your CDR element. Increase your CDR and your cost per Mb comes down even more. Be willing to buy transit from networks lower down the food chain and your cost per Mb will also plummet more.
The question was:
What benefits do we get for our cash by using an ISP?
I pay my provider £7.50 a month for broadband, a bit less than £300 to install or £100 per month to run.