In large volume, 2/10ths to 5/10ths a cent. That suggests large ISPs pay something less than a penny per gigabyte. If you use 139 gigabytes/month, that costs your provider something like $1/month. (Doubling transit costs gives you a rough estimate of the cost to the carrier, which also has to carry the bits to your local exchange.)
Akamai recently bid "$0.002 per GB delivered," according to Dan Rayburn, a streaming media expert I trust. Dan notes that's the lowest price he's ever seen for a content delivery deal, and other recent bids have been as high as half a cent. Prices have been driven down by large customers, including Apple, doing their own content carriage. Amazon is in the game today as well.
Hurricane Electric is advertising Internet transit at dozens of locations for $0.20/Mbps, which works out to a similar price.
The HE price requires a five year contract, so in effect is a forward price. The same probably applies to the Akamai pricing.
Small and rural carriers often pay much more, sometimes 20X higher. Those facing cartels also have higher costs. Most African ISPS, except the largest, face a cartel on backhaul and transit. (They also often face high national costs.) Akamai faces cartels that raise terminating charges in China and Australia.