Increasingly, the most popular iPhone apps are the free or lower-priced ones, according to mobile app store trend monitor Distimo.

Despite Apple’s keenness to put high-ticket apps like satnav apps on its Featured area, the average price of the most popular iPhone apps fell by 15 percent between December 1 and February 28, its monthly report says. The sharpest fall was in Australia (27 percent).

Europe spends most on its leading apps on average. The average price of the most popular apps there was $3.86 in February; the U.S. has the lowest average price of the most popular apps, $2.43. “This is due to the large number of turn-by-turn navigation applications covering different regions in Europe.”

That could change over time, with Nokia (NYSE: NOK) and Google (NSDQ: GOOG) starting to offer free turn-by-turn navigation.

Some apps are beginning to drop the download fee in favour of in-app subscriptions. But there’s still an interesting and growing market for high-priced downloads - I recently coughed up £19.99 for the official F1 2010 live timing and track position app.