Source: Apple.

Just days after T-Mobile (TMUS 0.09%) announced its latest JUMP On Demand program that allows new and existing customers to lease a new Apple iPhone 6s for as little as $5 per month, Sprint (S) answered back with its own ridiculously low iPhone leasing price -- of just $1 per month. 

Sprint's new plan falls under its current iPhone Forever plan, which allows users to lease iPhones for a cheap monthly price and then upgrade each time Apple releases a new phone.

But under the new offer, Sprint takes things a bit further. Starting on Sept. 25, new and existing customers can get a 16GB iPhone 6s for just $1 per month or an iPhone 6s Plus for $5 per month -- if you trade in an iPhone 6. The carrier said you have to go into a Sprint retail store to get the offer and that it's only good for a limited time, though the carrier doesn't say when it will end. Of course, if you want more storage on the device you'll pay a bit more per month. 

Here's a quick breakdown:

(monthly price)
(monthly price)
(monthly price)
iPhone 6s $1  $5.77 $10.53
iPhone 6s Plus $5 $9.77 $14.53

Source: Sprint.

For those without an iPhone 6, Sprint is also offering a $10 iPhone 6s leasing plan ($14 for the Plus) if you trade in an iPhone 5s. And if you trade in any other smartphone, Sprint will offer a $15-per-month deal to lease the 16GB iPhone 6s.  

How this differs from T-Mobile's latest offer
First off, Sprint customers will need to sign either a 21-month or 22-month lease agreement, depending on whether they're trading in a device. Meanwhile, T-Mobile's agreements are for 18 months, and offer the option to buy the device at the end of the agreement (Sprint's plan is a 100% lease).

Second, at signing, Sprint customers aren't required to pay anything as a down payment . T-Mobile offers the same $0 down payment option for the 16GB iPhone 6s, but charges $100 or $200 if you opt for the larger iPhone 6s Plus or more internal memory. 

One of the biggest differences is the trade-in requirements, though. For non-iPhone 6 owners, T-Mobile offers better pricing options. For example, a person trading in a Samsung Galaxy S5 to T-Mobile and leasing a 16GB iPhone 6s will pay $10 per month, but would pay $15 per month on Sprint's plan. For iPhone 5s users, T-Mobile's and Sprint's plans cost the same. 

Why Sprint needs this 
Recently, T-Mobile became the third-largest U.S. wireless carrier by subscriber numbers, and Sprint's trying to ensure it doesn't fall further behind. Through its network expansion and promotions, T-Mobile's managed to add 1 million or more customers each quarter for the last 10 quarters, and expects to add 2.1 million net adds for Q3 2015, the same it had in the previous quarter. 

While Sprint's network is still ranked higher than T-Mobile's in the latest RootMetrics report, Sprint's been fighting to bring customers back to its network. In fiscal Q1 2015, the carrier reported its lowest posted churn rate in the company's history and added 675,000 net additions. But postpaid phone losses hit 12,000, which means the carrier needs new incentives to build back its phone business. And it's hoping its new iPhone pricing can help contribute to that growth, while stealing away some potential T-Mobile customers at the same time.