Prices on mobile phone service plans fell an average of 4 percent in June, the largest decrease in U.S. wireless costs since last October, according to a new study. According to Econ One, an economic research and consulting firm, wireless prices fell in 24 of the top 25 cities surveyed--only the Washington, D.C., market saw rate hikes. The average monthly rate nationwide is $40.64. The most expensive markets include Los Angeles, San Francisco, Cincinnati and Kansas City; while Sacramento, Seattle, Phoenix and Atlanta were the cheapest.