The cost to acquire a loyal app user dipped 16% in September says Fiksu, the mobile app marketing platform provider. Fiksu defines a loyal user as someone who opens an app three or more times.

You may thank Apple for the dip in costs, as the much-anticipated iPhone 5 triggered app downloads to surge by 33% in the weeks following its Sept. 21 launch.

"A new device launch always presents an opportunity to cost-effectively acquire and engage new loyal users – and marketers are starting to anticipate this," said Micah Adler, CEO, Fiksu. "As we observed, brands held off on advertising in the run-up to the iPhone 5 launch, then jumped into the fray as the phones hit stores and mailboxes. Despite the increase in advertising volume and CPCs, marketers who went after these new users were rewarded with a nice drop in overall costs for acquiring loyal users."

The Fiksu Cost per Loyal User Index was $1.13 in September, down 21 cents or 16% from August’s $1.34. The last time the Cost per Loyal User Index came close to this was May 2011 when it was $1.10 (again thanks to Apple, says Fiksu, as a result of Apple's ban on incentive-based installs); then January 2012 when it fell to $1.14 after the holiday ad frenzy.

The Fiksu App Store Competitive Index (which measures the average aggregate daily download volume of the top 200 free U.S. iPhone apps) netted 4.07 million daily downloads in September, a slight uptick from 4.05 million in August. But there was a clear distinction in download activity before and after the iPhone 5 hit shelves. Downloads decreased by 3% in the weeks prior to iPhone 5's arrival, but jumped 33% in the weeks following.

The resulting traffic was good news for app marketers, with one Fiksu client experiencing a 20% increase in organic downloads and 35% revenue gains during the post-launch period.

The Fiksu Indexes measure monthly fluctuations in competition for rank in the Apple App Store and the cost to acquire loyal users.