Metro Phoenix home prices are headed for a new low, if they haven't already hit it. Median prices for the sale of existing homes have been falling since June, when a federal homebuyer tax credit expired and an increase in foreclosures helped drive down prices that had been steady for nearly a year. A new low would create a double dip in a market that has already been on a harrowing ride. Prices rose to about $250,000 during the boom of 2004-06 and then collapsed amid a mortgage crisis and an economic recession. They bottomed out at $119,900 in April 2009, according to the Information Market, a real-estate research firm.