According to NPD Group’s new Smartphone Track service, smartphone prices are falling even though many consumers originally considered paying more.

Among U.S. consumers who considered purchasing phones in the $200 to $250 price range, 64 percent ended up purchasing a phone for less than $200. Led by continued steady sales for Apple’s iPhones, the top five best-selling mobile phone handsets in Q3 in order were as follows: Apple iPhone 4, Apple iPhone 3GS, HTC EVO 4G, Motorola Droid 3Samsung Intensity II.

The share of U.S. mobile handset sales that were smartphones reached 59 percent in the third quarter of 2011, an increase of 13 percentage points over the same quarter last year. However, average selling prices for smartphones have declined for four consecutive quarters, reaching $135 in the third quarter.

Ross Rubin, executive director of industry analysis for NPD, said in a press release that there’s an opportunity for further revenue in the accessory business.

“Even as smartphone prices continue to decline, the accessory attachment rates for smartphones in Q3 was unchanged since last year,” Rubin wrote. “This indicates further opportunity for retailers to improve revenue numbers by focusing marketing efforts on selling more accessories.”

The top U.S. mobile accessories brands in Q3 were Zagg, Otter Products, Just Wireless, Motorola and Incase Designs.