Instead of paying a subsidized price for a handset with a two-year contract, the financing plans charge the full retail price for the phone spread out over a set period. Once it's available later this month, a 16GB iPhone 6 will fetch $649.99 retail, or $199.99 with carrier subsidies. That works out to $32.50 per month under a Verizon Edge 20-month plan, or about $27.09 under AT&T's Next 18, which spreads payments over 24 months.
Users get a price break of $10 to $25 per line on their monthly service plan, and can opt to trade in their phone any time after a set point. For regular upgraders, they operate more like a rental than a purchase.
"That's the flexibility you have," said Kirk Parsons, senior director of telecom services at J.D. Power & Associates. Verizon sets the bar at a minimum 30 days and 60 percent of the phone price, and with AT&T's plans, it's 12 or 18 installments (about 60 or 75 percent of the price).
T-Mobile users also have options, through several services. The carrier's Simple Choice plans let customers pay for handsets over a 24-month period. A $10-per-month Jump! add-on allows upgrades once 50 percent of the handset price is paid, and also includes a protection plan.
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If your handset purchases tend to be more buy-and-hold, contract plans or prepaid are still likely to be the more cost-effective options, said Parsons. But handset financing and early upgrade options can yield savings for early adopters planning a mid-contract upgrade.
Over a two-year contract, for example, a regular AT&T customer and iPhone devotee might pay $2,810. That's $200 for the first, subsidized 16 GB iPhone, $650 for a mid-contract upgrade to Apple's latest, a $40 upgrade fee and $1,920 (before tax) over the 24-month period for a single line with unlimited talk and text and 2GB of data. An AT&T Next 12 customer, meanwhile, might pay $2,340—$390 in installments for the first phone, and then $390 for the upgraded one, plus $1,560 in service charges. Estimated savings: $470.
Even assuming that the contract customer opts to sell his first in-contract phone on the secondary market—sites such as Gazelle and NextWorth offer roughly $300 for one in like-new condition—the savings gap is still nearly $200 in favor of the early upgrade plans.
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But paying off a handset over time isn't a good option for butterfingers or those who might be tempted to switch carriers. Because you owe money on the phone, the costs are bigger if it's damaged, stolen or lost. "Then you're committed to buying a new device at full price and still pay off the one you had," Day said. Switching to a different provider? You'll have to pay off the remaining balance owed.
Carriers do offer protection plans that can cover the tab for a damaged phone. That cost, roughly $10 per month, cuts into savings—and you may still have a deductible to cover costs. (In the case of T-Mobile jump, it's $175.)
UPDATE: This story was updated to reflect details about the new iPhone 6 shared during Apple's announcement.
—By CNBC's Kelli B. Grant