A Monthly Income Building in SimCity 4 is a special type of building that adds to the Monthly Income you get instead of Monthly Expense. Most people that are at the border line with Income and Expense may want to add these 5 buildings to increase the income your city will make. The 5 listed Monthly Income Buildings are listed below. -Federal Prison -Army Base -Toxic Waste Dump -Missle Silo/Launch Pad -Area 5.1