Sprint seems to be the cheapest option for buying Apple's latest iPhones in the United States. Sprint's offer for the 16 GB iPhone 6S with a basic data, calling and text plan costs between $3 and $10.50 less per month than a competing offer from the other three top U.S. carriers.

Apple itself added pressure to U.S. carriers by offering its own financing scheme for an unlocked iPhone, which gives a customer the liberty to switch between carriers, rather than be tied down by a contract with one carrier.

However, Sprint's plan works out to be cheaper, on a per month basis, than even buying the iPhone from Apple and going for the cheapest wireless plan on offer.

Sprint is charging $22 per month for the 16 GB iPhone 6S, under a 21-month lease. That along with a basic talk, text and data plan, worth $45, will cost a customer $67 per month.

A T-Mobile US Inc plan similar to Sprint's will cost about $70 per month – $20 for the phone with an 18-month agreement and $50 for a basic wireless plan.

Verizon's plan costs about $77.08 per month, $27.08 for the iPhone over two years and $50 for a wireless plan.

AT&T's plan, Next12, costs about $77.50 per month, $32.50 for a 20-month phone contract and $45 for a wireless plan.

Under Apple's offer, the iPhone 6S will cost $32.41 per month. Paired with a basic $45 wireless plan, among the cheapest available, the phone would cost $77.41 per month.

Regarding upgrades, Sprint allows users to upgrade every time a new iPhone is launched, and charges an activation fee.

T-Mobile allows a free upgrade at any time, while AT&T offers the option of a free upgrade after 12 months.

Verizon's plan also permits an upgrade at any time, but requires customers to pay off the remaining cost of the iPhone.

On the otehr hand, Apple's financing plan includes AppleCare+, a warranty program that provides two years of hardware protection, instead of the standard one year, as well as coverage for two incidents in cases of accidental damage.