Manhattan condo developers, in a construction revival after the credit crisis, are raising prices on their unbuilt units as often as twice a month as buyers return from the housing slump to find there’s little on the market. The borough’s inventory of homes for sale fell to the lowest in at least 12 years in the fourth quarter, according to appraisal firm Miller Samuel Inc. Sales, meanwhile, were the second-highest in a decade in 2012.At least 21 new condominium buildings in Manhattan, comprising 1,188 units, increased some of their asking prices last year, according to New York State Attorney General Eric Schneiderman’s office, which reviews details of condo plans. The market has heated up to the point where buyers are putting deposits on units that haven’t been finished, a return to boom- era practices, said Shaun Osher, chief executive officer of the New York brokerage CORE, which is advising Walker Tower on prices.“Ultra-low inventory, international demand and rising domestic confidence has really pushed pricing,” said Kelly Mack, president of Corcoran Sunshine Marketing Group, the new- development sales unit of New York brokerage Corcoran Group. “We’re seeing quick decision-making like we haven’t seen in years,” she said of buyers[...]At Walker Tower, formerly offices for Verizon Communications Inc., the value of all apartments climbed 5.8 percent since May to $533.2 million, according to amendments filed with the attorney general. The 24-story building, at 18th Street near Seventh Avenue, began sales in June, and residents are slated to move in later this year, said Osher of CORE.“We’re going to see a lot more people stepping up to the plate and buying off floor plans,” Osher said. “This is really a sign of what’s happening: Incredible demand plus low inventory.”A three-bedroom apartment on the 11th floor of Walker Tower is being marketed for $9.25 million, or 16 percent more than its $8 million price tag in May, according to a Jan. 15 filing with the attorney general’s office. The full-floor penthouse on the 24th floor is listed for $55 million, up from $50 million in May. The 5,955-square-foot (553-square-meter) unit has five bedrooms, five bathrooms and comes with 479 square feet of outdoor space.Citizen TowerAnother development in Chelsea, the Citizen, has increased prices 62 percent since sales began last April, according to attorney general filings. The newly constructed tower was built on a land parcel acquired by developer Anbau Enterprises Inc. on Sept. 15, 2008, the day that Lehman Brothers Holdings Inc. filed the largest bankruptcy in U.S. history, ushering in a freeze to the Manhattan sales market.[...]There were 4,749 apartments on the market in Manhattan at the end of December, a 34 percent decline from a year earlier, according to Miller Samuel. Sales last year rose 3.4 percent to 10,508 transactions, second only to 2007 in the past decade.[...]The average price of a Manhattan condo sold in the fourth quarter was $1.87 million, up 5.4 percent from a year earlier, according to Miller Samuel.