The following list of resources will provide you with some of the information you need to build your case. Also be prepared to conduct a general web search using keywords you find in the text of the listed resource sites.
This site can give you a broad overview of information. It also has a searchable database and extensive section of related links.
This site gives some very detailed information regarding actual monetary benefits of urban trees.
Article by the Virginia Cooperative Extension.
TreeFolks of Austin, TX give a short but succinct list of reasons for planting trees.
This site offers advice on how to determine the value of trees and also gives some practical guidelines for planning with trees.
This is the US Forest Service website for the Northcentral region. It deals specifically with community & urban forestry issues.