This iPhone upgrade season is turning into a merciless price-cutting war. After Apple (AAPL 0.71%) surprised the market with its own iPhone Upgrade Program, which potentially marginalizes carriers while outsourcing credit risk, T-Mobile (TMUS 0.50%) took the pricing lead by announcing a promotion for its new Jump! On Demand leasing program that offers a new entry-level iPhone 6s for $0 down and $20 per month. That compares to Apple's pricing that starts at over $32 per month, in part because it includes the $130 worth of AppleCare+.

But consumers who are willing to trade in an iPhone 6 can bring that monthly cost down to a bargain $5 per month. You might spend more than that on a cup of coffee. Trading in older phones with lower values will translate into lease payments of between $5 and $20 per month, depending on the model.

Having just recently ceded the title of No. 3 carrier to T-Mobile, Sprint (S) is determined to fight back. The now-No. 4 carrier has just announced its own incredibly cheap iPhone leasing program at just $1 per month when customers trade in last year's model.

The fine print could be where the differences lie
Beyond the headline monthly fees that the battling carriers are touting, investors and consumers should really look to the fine print.

For starters, after the 18-month lease term is up, customers can always exercise a purchase option to own the device outright. For T-Mobile, the Un-Carrier offers the device for $125 less than the full retail price, with all lease payments made being applied toward that total. This $125 is essentially the value of the promotion that T-Mobile is offering. Carriers get an estimated $25 concession per unit from Apple, so the net cost of this promotion should be just $100.

However, T-Mobile is essentially valuing your iPhone trade-in at a low $270 (which is the difference in the monthly fees over 18 months), and it will turn around and sell that used phone at a profit anyway to help recover its promotional expense.

Sprint similarly offers a purchase option, but does not specify how much it is. I called both Sprint's sales and service lines and was sadly unable to get a definitive answer. Sprint has long had a reputation for terrible customer service. Neither of the two representatives I spoke with were able to answer my question. The first rep transferred me to the second rep, which placed me on an extended hold before I was disconnected altogether. The price of this final purchase option could tip the scales, if Sprint is merely asking for less now in exchange for more later.

Say what you will about the outgoing subsidy model, but at least the math was simpler, albeit less transparent.

The real winners here: Apple and you
As Sprint and T-Mobile compete on price while the larger carriers attempt to stand their ground, the ultimate winners here are the consumer and Apple. Consumers now have an incredible amount of choice when it comes to buying a smartphone, and those choices are coming at lower and lower price points.

All the while, the net result is that Apple gets to sell more iPhones and is enjoying accelerated upgrade activity, and it doesn't even have to dilute its brand with aggressive discounts since the carriers are willing to compete on that front.