You'd think a megabyte would cost the same no matter what device used it. You'd be wrong Cellular carriers inhabit a Twilight Zone universe. For example, they offer “unlimited” data plans that are, in fact, limited and subject to “reasonable use,” meaning whatever they choose it to mean at the particular moment. And they often charge very different prices per megabyte of data, based on the device you happen to be using — an odd result of their move to consumption-based billing, which was ostensibly meant to implement fair costs for all users, rather than let data hogs get a nearly free ride at everyone else’s expense.Dubious differences within carrier plans The curious pricing scheme that is emerging leads to the following oddities with carriers’ plans.AT&T charges smartphone users 8 cents per megabyte for 200MB of data, but 3G USB modem and MiFi users pay 18 cents per megabyte for the same 200MB. Corporate smartpone users pay 2 cents per megabyte ($45 per 2GB), but USB and MiFi users fork over just 1 cent per megabyte ($60 for 5GB). And AT&T charges pay-as-you-go tablet users $15 per each additional 200MB of usage, but it charges pay-as-you-go smartphone users $10.24 per additional 1MB — that’s 128 times the price per megabyte. T-Mobile charges smartphone users less per megabyte than it does tablet and USB/MiFi users. You pay 5 cents per megabyte for the basic smartphone data plan ($10 per 200MB) but 10 cents if you use one of the other devices ($25 for 250MB). It makes sense that the company’s unlimited data plans’ pricing favors smartphone users ($30 versus $40), as tablets and laptops are likely to eat up more data than smartphones. But when you’re paying for a set amount of data, the 2:1 cost difference doesn’t make sense.Verizon Wireless charges twice as much per megabyte for additional data than it does for the original data: A smartphone user pays 1 cent per megabyte ($20 for 2GB) for the first 2GB, but 2 cents per megabyte ($20 for 2GB) for the next 2GB. Sprint has a greater disconnect in its 3GB pricing: 1 cent per megabyte in its initial tablet and USB/MiFi plans ($30 for 2GB and $60 for 5GB), but 5 cents per additional megabyte. Sprint charges netbook users less for additional megabytes (5 cents) than for the initial megabytes (8 cents, based on $40 for 50MB). However, that makes sense since the netbook is sold at a subsidized price, so the initial $40 includes a payback amount for the device’s subsidy.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. Have fun navigating the carriers’ 3G pricing! To help you do so, I’ve put together a pricing table, current as of Dec. 2, 2010.Monthly plans AT&T Sprint T-Mobile Verizon Wireless Smartphones Personal use $15 per 200MB $0.08 $30 “unlimited” — $10 per 200MB $0.05 $20 per 2GB $0.01 Personal use $25 per 2GB $0.01 — — $30 “unlimited” — — — Corporate use $45 per 2GB $0.02 $40 “unlimited” — — — $40 per 2GB — All use additional MB same as above additional MB $0.00 additional MB $0.00 $20 per additional 1GB $0.02 Tablets All use $15 per 200MB $0.08 $30 per 2GB $0.01 $25 per 250MB $0.10 $20 per 1GB $0.02 All use $25 per 2GB $0.01 $60 per 5GB $0.01 $40 “unlimted” $0.00 $35 per 3GB $0.01 All use — — — — — — $50 per 5GB $0.01 All use — — — — — — $80 per 10GB $0.01 All use additional MB same as above additional MB $0.05 additional MB same as above $10 per additional 1GB $0.01 USB modems/MiFi All use $35 per 200MB $0.18 $60 per 5GB $0.01 $25 per 250MB $0.10 $40 per 250MB $0.16 All use $60 per 5GB $0.01 — — $40 “unlimted” — $60 per 5GB $0.01 All use additional MB $0.05 additional MB $0.05 additional MB same as above additional MB $0.05 3G netbook All use $35 per 200MB $0.18 $40 per 500MB $0.08 — — $50 per 5GB $0.01 All use $60 per 5GB $0.01 — — — — $80 per 10GB $0.01 All use additional MB $0.05 additional MB $0.05 — — $10 per additional 1GB $0.01 No-commitment plans AT&T Virgin Mobile T-Mobile Verizon Wireless Smartphones All use $15 per 200MB $0.08 — — $10 per 100MB $0.10 — — All use additional MB $10.24 — — $20 per 300MB $0.07 — — All use — — — — $50 per 1GB $0.01 — — Tablets All use $15 per 200MB $0.08 — — — — — — All use $25 per 2GB $0.01 — — — — – — All use additional MB same as above — — — — — — All use — — — — — — $80 per 10GB $0.01 USB modems/MiFi All use — — $10 per 100MB $0.10 — — $15 per 100MB $0.15 All use — — $40 “unlimited” — — — $30 per 300MB $0.10 All use — — — — — — $50 per 1GB $0.05 All use — — — — — — $80 per 5GB $0.02 This article, “The strange, unpredictable pricing of 3G data plans,” was originally published at InfoWorld.com. Read more of Gruman et al.’s Mobile Edge blog and follow the latest developments in mobile technology at InfoWorld.com. Technology IndustryVerizonT-MobileSprintAT&T