Apple doesn’t chat too much about the iPhone service fees AT&T shares with it. But the good folks at the Financial Accounting Standards Board (who make rules for how companies keep their books) have forced it to give us some clues. The accountants say that if someone promises to pay you money in the future (as AT&T does for each iPhone activated), you have to report this “deferred revenue” to investors.

All this makes Apple’s financial statement into a rather complex algebra problem. Gene Munster, an analyst at Piper Jaffray, has tried to do the math.

His bottom line is rather shocking: AT&T appears to be paying $18 a month, on average, to Apple for each iPhone activated on its network. That adds up to $432 over a two year contract. In other words, Apple will receive $831 for each iPhone it sells. (It’s a little less for iPhones sold in AT&T stores.) He can’t tell, of course, what makes up that average. Apple may, for example, receive higher payments for customers new to AT&T or those who buy more expensive monthly plans.

Mr. Munster says he was surprised by the numbers. He previously had estimated that Apple received $6.50 a month from AT&T. A carrier will typically subsidize the price of a high-end phone by about $200, he says. With the iPhone, AT&T is spreading the payments out over time, but ultimately putting up twice the typical amount.

The numbers will be harder to figure out from here on out, because Apple has started selling iPhones in Europe, where it has presumably different arrangements with carriers in Britain, Germany and France. But Mr. Munster says he figures the payments from them will be at least as much as from AT&T. And that certainly is consistent with reports that Apple has been demanding as much as 40 percent of the monthly fees from European carriers.

Still to Mr. Munster, this is incredibly bullish news for Apple investors. The higher revenue, of course, compounds Mr. Munster’s already optimistic view of the IPhones sales prospects. He figures Apple will sell a total of 3.4 million iPhones this year, 12.9 million next year and 45 million in 2009. The boom, he suggests, will come from increased features such as navigation and faster data speeds, as well as a lower price. He figures the average price of the iPhone, now $399 in this country, will drop to $300 in two years.

Mr. Munster assumes that the payments from carriers will then fall to $9 a month. As a result of this analysis, his estimate of Apple’s 2009 revenue increases from $36.2 billion to $42.8 billion. And the company’s net income per share increased by 14 percent. The bottom line: Mr. Munster’s target price for Apple’s stock, which today sells for around $184, goes to $250. (He had it pegged at $220).

All this shows how much incentive Apple has to maintain its exclusive deal with AT&T rather than to sell unlocked phones or cut deals with multiple carriers. AT&T is finding that an exclusive on the iPhone draws in new customers and lures existing ones to buy more expensive data plans. Carriers will keep paying Apple big kickbacks so long as its phones still stand out from the pack.