Apple's

(AAPL) - Get Report

App Store has certainly fired up the public's imagination, with more than a billion downloads in less than a year.

Just how much of this has translated into revenue, however, remains a mystery, and one Silicon Valley expert estimates that Apple has made no more than $45 million from the digital store.

Launched last July, the App Store is seen as a key weapon in

Apple's

iPhone and iPod arsenals, although the firm is staying tight-lipped on specific revenue details. The notoriously secretive tech bellwether has not even revealed the proportion of paid to free applications.

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Cue Jeremy Liew, a managing director at venture capital firm

Lightspeed Venture Partners

, who has been trying to figure out just how much money Apple has made from all those downloads.

"Although it's hard to come by the definitive ratio of paid to free apps, talking to industry participants I got estimates in the 1:15 to 1:40 range," he wrote in a blog posting this week. "So that suggests that between 25 million and 60 million paid apps have been sold."

Liew pointed to a recent survey of iPhone apps by research firm

O'Reilly

, which said that the mean price for a paid application is $2.65.

"Multiplying this by 25 million to 50 million applications, that suggests that the cumulative revenue from iPhone apps is around $70 million to $160 million," he wrote. "Apple gets 30 percent of this, so Apple has probably made around $20 million to $45 million from the billion iPhone apps downloaded."

The figures, although estimates, suggest that the App Store accounts for just a tiny slice of Apple's overall sales. The Cupertino, Calif.-based firm posted revenue of $8.16 billion at the end of its recent third quarter, boosted by strong demand for iPhones.

There had been

speculation

that the App Store could become a billion-dollar business by the end of 2009, although Liew's figures suggest a much more modest return.

Apple has not yet responded to a request from

TheStreet.com

about its App Store revenue.

Money aside, though, Apple has again proved its ability to anticipate and tap into a consumer technology trend with the App Store, which is growing rapidly. For example, it now offers more than 35,000 applications in the store, compared to just 15,000 at the end of its December quarter.

Although still in its relative infancy, the App Store has already

spawned imitators

from

Microsoft

(MSFT) - Get Report

,

smartphone rival

Research In Motion

(RIMM)

and a

handful

of other challengers.

Still, Apple is probably not losing sleep about its App Store revenue, according to the

AppleInsider

Web site. "Similar to its stance on the iTunes music and movie stores, Apple has maintained that the App Store isn't meant as a profit generator and is instead a means of attracting customers to the iPhone and iPod touch, where the majority of the profit exists," it said.