There are three types of benefits:
Tangible Benefits,
Intangible Benefits, and
Additional Benefits
Tangible beneifts are those that you can attribute a number to - numbers like $ saved or hours of productivity improvement. But for an ROI Calculation, you need to reduce it to $$. For productivity savings, or reduction of maintenance time, determine an appropriate labour rate and x by the hours saved.
Intangible benefits are nice to have, but have no economic benefit. Easier to use is often an intangible.
Additional benefits are those benefits that you wont get just from doing the project - they only become benefits for future projects - adding extra unneeded capacity for example.
Once you have calculated the benefits, you then have to calculate the costs - how much will it cost to implement(capital cost of server and labour cost of the project).
Then you can either calculate an annual rate of return on investment, or my preference - a payback calculation -the payback calc tries to determine how many months before the benefits exceed the costs, typically on most IT projects from 6 months to 2 years. Its a more demonstrative calculation.
James