The number of iPhone app downloads crashed in March, but the cost of marketing those apps held steady according to the latest Fiksu index report.

Mobile app downloads fell by 30 percent, or 2 million downloads, in March and returned to pre-iPhone 4S-launch levels. The cost of acquiring a loyal user (one who opens an app three times) fell slightly from $1.31 per user in February to $1.30 per user in March. That is a surprise since Japan’s new market entrant Gree supposedly spent heavily (a force that could driving marketing costs up) during March when it revealed its first U.S.-made apps.

The Fiksu App Store Competitive Index (which measures the average aggregate daily download volume of the top 200 free U.S. iPhone apps) dropped from 6.35 million downloads in February to 4.45 million in February. That’s a little alarming since Apple is a leader in the app market.

“With the novelty factor of the iPhone 4S launch and the holidays well behind us, and no other events in March to spark discovery, March’s download dip was expected,” said Micah Adler, chief executive of Fiksu. “An unexpected contributing factor could be the decline in the use of robotic install tactics by app marketers responding to Apple’s new policy.”

Apple announced in February that it would enforce an existing guideline that prohibited the use of bots to market apps and would frown upon other forms of chart manipulation.

Adler said that the “decline in competition and steady costs definitely presented app marketers with a ranking opportunity in March, driven largely through the cost-effective conversion of organic users into loyal users.”

Fiksu, based on Boston, sourced its data from more than 21 billion mobile app actions.

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