Busines cases are so simple: give the decisionmaker what they want
When you are building a business case for ITIL (or anything), it doesn't matter if it is intangible Value or $$$Return that the proposal is offering if that is not what the business wants at this time.
Here is an excellent white paper from Pink Elephant on VOI "Value of Investment" as compared to ROI "Return on Investment". The only thing I would add to it is this one point: offer them what they want to hear.
If the company is currently on a desperate cost-cutting drive to avoid going under, then all the intangible value in the world isn't worth a vendor's promise. On the other hand if the new CEO is driving the company to new strategic heights and all the senior management have just returned from an off-site lovefest in Hawaii, then you can use the right words and you are in without a real solid cost-saving in sight.
Of course in the unlikely event that the ITIL project has a tangible ROI with payback in under 18 months, then it doesn't matter whether management are in kaftans or khaki right now, the business case is likely to get approved. But it would be smart to make sure by still framing it in the right terms.
So don't start from "what ITIL can offer you" and heap a pile of irrelevant arguments onto the decision maker. Start from "What you want to hear, Mr/Ms Decisionmaker", then show how you can deliver it. And if you don't know you should start by finding out. Eh? Why, by asking them of course.