Mig, that is the inherent issue with building all business cases. How do you know what will happen in the future?
With your team:
1. Capture your set of assumptions; these are things you think are true, based on your intuition, expertise in your market place, theory, and speculation.
2. Capture what you know for a fact; these things could be similar results from a similar implementation either at your org or another, customer suggestions or feedback, etc.
3. Capture the questions you need answered; many of these will come from your set of assumptions, too.
Don't get married to your idea. Look for evidence NOT to do it versus evidence to do it.
I think the largest obstacle in building a case is that most people are uncomfortable dealing with uncertainty. Get comfortable working with ranges of results, costs, and risks, and with probabilities. Build a non deterministic pro forma versus a deterministic one and simulate if you are able.