The carriers did not respond to my requests for an explanation why the price per megabyte varies based on the device or why -- in Verizon's and Sprint's cases -- some additional megabytes cost more than the initial megabytes. One answer could be to discourage use by charging more per megabyte with increasing access -- except that each of their plans with set allotments get cheaper per megabyte the more you consume.
There's a reason for that: Most people will never require all the data they buy, so in lowering the cost per megabyte apper for heavy users, the carriers fool people into paying more than they need to. It's an old trick, as is the practice of entry-level cellular plans having not quite enough minutes or megabytes, forcing customer to the next costlier level.
Price differences across carriers' plans
The varying prices per megabyte for different devices by the same carrier is certainly odd. Not so unusual is the fact that the price per megabyte can range wildly across carriers, as carriers try to favor certain services or devices in hopes of attracting the most profitable market segments. If you're looking at adding 3G devices to your personal or business mix, keep these differences in mind:
AT&T, Sprint, and Verizon offer similar pricing of about 1 cent per megabyte for smartphone users: AT&T charges $25 for 2GB, Verizon charges $20 for 2GB, and Sprint and T-Mobile charge $30 for "unlimited" data. T-Mobile is the cheapest carrier if you want limited data: $10 for 200MB (5 cents per megabyte), versus AT&T's $15 for 200MB (8 cents per megabyte).
If AT&T or Verizon catches you using your smartphone to connect to corporate email (Microsoft Exchange or Lotus Notes), you'll pay $20 more per month for 2GB at AT&T and $15 at Verizon. For 2GB, that makes AT&T's corporate pricing $45 and Verizon's $35. And note that AT&T's $15-per-200MB option is not available for corporate use; you'll have to move up to the 2GB plan if discovered accessing corporate email. Sprint tacks on $10 per month if you use a BlackBerry with the BlackBerry Enterprise Server (BES), but doesn't charge more if you manage to connect one of its Android smartphones to an Exchange or Notes server. Why corporate email should cost more per megabyte than personal email is also an oddity.
Although their tablet plans differ, all four carriers charge essentially 1 cent per megabyte. (T-Mobile and AT&T also offer low-cost plans that cost 10 cents and 8 cents per megabyte, respectively.) The key difference is that Sprint charges 5 cents per additional megabyte, so your overage charges are unpredictable, whereas the other carriers charge a flat rate per additional tranche of data. But as Sprint's basic plan offers 2GB of data (for $30, or 1.5 cents per megabyte), going over is probably not a risk unless you're streaming TV and movies, which can take 1GB to 2GB each. Note that Sprint is the only carrier to offer tablet 3G service without requiring an ongoing commitment; you can start and stop whenever you want -- perfect for the occasional traveler.
The best deal for a 3G USB modem or a MiFi service comes from T-Mobile, which charges $40 for "unlimited" usage. AT&T, Sprint, and Verizon all charge $60 for 5GB (1 cent per megabyte) and 5 cents per additional megabyte. If you don't want to sign a contract, Sprint's Virgin Mobile USA subsidiary charges $40 for unlimited 3G data with no obligation to renew. It also has a $10-for-100MB no-commitment option for a one-day trip -- it's cheaper than paying for hotel and airport Wi-Fi.