It’s common to hear the Apple iTunes music store described as a “loss leader” for the company; that’s an image Steve Jobs has encouraged, as well. Billboard recently crunched the numbers and discovered, as you might expect, that that’s not the full story. (I can’t give you a link because the magazine’s incomprehensible web site can’t find it for me.)

Ed Christman, the Billboard’s retail columnist, figures Apple sold 1.7 billion tracks last year out of the store, which makes for about $1.9B in revenue, taking into account slightly higher prices overseas. Apple turns over 70 percent of that to the music companies, leaving $570M for the company.

Apple doesn’t break down its figures for Christman as to marketing and technical expenses; so he looks at how Amazon works and concludes the company spends about $180M a year on such things, meaning that Apple might make about $390M profit on the iTunes operation, before depreciation, amortization or taxes.

Four hundred million dollars in profit would be a lot of any company, including Apple—until last year, when its profits rocketed up (65 percent and more, in some quarters, to more than $4B for the year) and its stock price doubled.

Still, in the end it seems that the iTunes store represents about 10 percent of the company’s profit. That’s not decisive, but it’s not chump change, either. And the nature of digital sales is such that, as the stores sales grow, Apple’s profit margin will grow as well.