Not only are those lots a net user of city funds, they are also eye sores that inhibit potential builders and home owners from considering them in their building and expansion plans. Those lots and vacant buildings are also sources of crime and corruption, running current and potential tax payers away. The simple solution is to convert those lots to tax producers for the city, not to be net users of precious city funds. Mayors, councilmen, and others in many cities have been trying just that for years. It is not an easy task but if a city is to survive it must invest in bringing good tax paying citizens and businesses back to the city.
Cities condemn buildings, cities demolish buildings and cities have city planners.
They also have an abundance of prime property within its city limits. All the main ingredients are there. All that needs to be done is to get them working together. For a developer or home builder or new business to be initially attracted back to the inner city, the city must provide a safe, attractive area that others also want to build around. Making an area attractive could also have the side effect of also making it more safe.