3 Step Process to calculate the ROI (Return on Investment) on IT Projects
You may also read Missing SAP ROI's ? 
Step 1 - Computing Total Investment
Step 2 - Computing Total Return
Money Return -
Increased Cash-Flows due to either Increase in Revenues or Decrease in Expenses directly or indirectly attributable to the IT Project.
Time Return - Time saved by Organisation's People in their Personal lives expressed in Money terms after Project Implementation is completed.
This has to be estimated for whatever is shorter - person's work duration with the Organization after the IT project start or the IT project's estimated life.
Knowledge Return - Increase in the Knowledge of the People within the Organization due to their Expertise gained due to the Exposure to the IT Project Implementation.
Brand Return - Increase in Value of the Organisation's Brand - It's Image in the minds of it's critical Stake Holders (Previous, Current and Future). These are The Organization's Owners, Customers, Suppliers, Employees, Regulators, Competitors and the rest of Society.
Step 3