That number, $2.4 billion, represents AdMob's estimate of how much users are roughly spending on an annualized basis on paid apps from the iTunes store. These figures are part of AdMob's July, 2009 mobile metrics report. Much of the report is in turn based on a mobile survey of 1,000 users (see additional data). Some of the top-level findings are as  follows (emphasis added):

• Android and iPhone users download approximately 10 new apps a month, while iPod touch owners download an average of 18 per month
• Upgrading from the lite version was the top reason given when users were asked what drives them to purchase a paid app
• iPhone and iPod touch users are twice as likely to purchase paid apps than Android users.
• Users who regularly download paid apps spend approximately $9 on an average of five paid downloads per month

Based on these figures AdMob extrapolates and estimates the value of the iPhone apps market and that of the Android Market for paid apps:

• iPhone and iPod touch users are twice as likely to regularly download paid apps as Android users.  19% of Android users regularly download 1 or more paid apps a month, compared to 50% of iPhone users and 40% of iPod touch users.
• These users spend roughly $9 a month and download 5 paid apps per month.  Dividing the average amount paid per month by the average number of paid apps downloaded yields an average app purchase price of about $1.80.
• App Store paid market is nearly $200 million per month versus $5 million per month for the Android Market

Here are some of the charts accompanying the report:

And among smartphones on its network, AdMob reports the following mobile Web activity (US):

• iPhone/iPod Touch: 60%
• RIM: 13%
• Android: 12%
• Windows Mobile: 6% 

Compare with last month's data:

• iPhone/iPod Touch: 64%
• RIM: 14%
• Android: 9%
• Windows Mobile: 6% 

Compare July global share:

• iPhone/iPod Touch: 45%
• Symbian/Nokia: 33%
• RIM: 7%
• Android: 7%
• Windows Mobile: 4% 

Conclusions:

• The mobile Internet is in the US still mostly about the iPhone though Android is gaining
• The iPhone paid apps market is big and getting bigger
• Users are quite willing to pay for apps at relatively small price points
• The free + paid app strategy seems like it's paying off (literally) for developers

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Related: Earlier this month we reported on earlier data from AdMob on app downloads and user behavior.

Update: Developers question the figures/math here.