If you use the end user cost you can get a high end estimate for total cost of 3g service. $30 for 2GB of data makes $0.015/MB * 18.1 MB file makes for a total cost to user of $0.27 for delivery.
Sure, on paper you get 2GB for 30$, but really your paying 30$ to be part of a pool of people that use below 500MB on average and some of them pay a lot more than that in overage feels for downloading 2.01+GB.
A more realistic number might be 15$ for 200 MB with the risk of overage fees = 1.35$ for 18.1MB. Amazon may pay more per MB than you do because of that lack of subscription good for life of the device thing or less, but they also have overhead to track that etc.
The fact that you can get 2 GB of data for $30 doesn't mean that you can get 18 MB of data for $0.27. Most people paying $30 for a 2 GB data plan are using far less than 2 GB of data.
