The city has spent nearly $50 million since 2004 demolishing buildings owners have left to rot, but weak collection efforts have largely left taxpayers stuck with the bill, Detroit officials acknowledge. But Mayor Dave Bing says he's about to get tough, promising to sue walkaway owners or sic collection agencies on them. After years of failing even to send bills for costs, the city soon plans to begin filing suits against the owners of the 40 most expensive demolitions of the past six years, mostly apartments or businesses, which run more than $25,000 apiece.