When I worked for a large corporation I was frequently tasked with building a business case without a budget, that is, I wasn’t able to hire any consultants to assist me. In some cases deadlines were relatively short so it was fairly difficult to get it completed. When the Internet came around more than once I was saved by people willing share business cases they had developed. Therefore I have uploaded a economic impact model that comprises two documents, an excel spreadsheet and word document that should cover the basic needs of a user. I have other more sophisticated models besides this one (for example, a business process and knowledge management re-engineering model that compares the economics of the current state versus the future state) but for the majority this should suffice to help you get started. If you find it useful just leave me a comment.
It can be downloaded here.
Update 20120801: Fixed the link again