Although many imagine the App Store a cash cow for Apple, a detailed examination has revealed that the iPhone maker may only have earned between $20 million and $45 million from the store for its first billion downloads — a figure the company is likely more than happy to accept.

Lightspeed Venture Partners’ Jeremy Liew made the calculation after learning that there’s typically a ratio of 15 to 40 free apps for every one paid example and after discovering an O’Reilly estimate which determines that the mean price for an app is about $2.65.

At the time Apple crossed the billion app mark, this would have resulted in about 25 million to 50 million downloaded paid apps and produced raw revenue of between $70 million to $160 million. But because Apple only takes 30 percent of that revenue for itself, the company has only earned $20 million to $45 million itself. That could be cut in half again if the weighted average is actually at a lower figure such as $1.50, Liew says.

The calculation would have App Store purchases account for an extremely small fraction of Apple’s iPhone revenue to date; if it were put up against Apple’s early 2009 revenue alone, it would represent less than 3 percent.

So Apple isn’t exactly raking in the dough from the App Store as much as many may think, which could partially explain the lateness of payments to providers recently.