Sprint Corp. (NYSE: S) announced a new plan that costs $60 a month for unlimited data. T-Mobile US Inc. (NYSE: TMUS) has one that costs $80. AT&T Inc. (NYSE: T) and Verizon Communications Inc. (NYSE: VZ) have not been as aggressive because they hold such a large part of the market. But if the two smaller companies begin to eat into that market share, the two larger firms will have to offer less expensive plans as well. The race toward the bottom of what wireless service providers charge for data plans has accelerated. At some point, prices may plummet toward zero, as the companies in the industry try to offset extremely low data fees meant to grab consumers with other charges customers have to pay. In the meantime, margins at these companies will be crushed.

The anxiety about margins has started to show in stock prices, although these have been affected by a slowdown of M&A in the industry as well. A deal that might have combined Sprint and T-Mobile has ended. Nevertheless, Sprint’s stock trades very close to it 52-week low and is down 33% in the past six months. T-Mobile’s share price is down 7% over the same period. Verizon and AT&T have underperformed the S&P 500 during that time.

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Some avenues wireless carriers might use to supplement data revenue are likely already closed to them. Most of the industry offers unlimited talk and texting. The same problem exists with unlimited messaging in some markets. And there has been a proliferation of plans for free cloud storage. For example, AT&T offers 50GB of storage in its AT&T Locker, which is primarily used for photos.

Another chance to recoup lost money from data plans is the price of smartphones themselves. Unfortunately, carriers offer many less popular phones for free, or close to it. And the carriers pay more for products like Apple Inc.’s (NASDAQ: AAPL) iPhone than they charge their customers for them. This is meant to attract new customers, or hold existing ones. The most popular smartphones, in other words, are “loss leaders.”

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All of this is to say that what is left to buttress wireless carrier revenue is the base cost of “smartphone lines,” which is the charge for operating a wireless device on the network itself. Customers used to be locked into these plans for two years and had to pay penalties to get out of them. That is less and less the case. AT&T now charges $65 a line without a long-term contract. In exchange, the customer gets unlimited talk and data and 2GB of data. Only AT&T knows exactly what it makes on this plan, or perhaps loses.

Just a year or two ago, the wireless business was supposed to be the financial future of telecoms as they lost landline business. The math of the industry is no longer that simple, particularly as the customer charges for data plans move toward zero.

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