You need two types of metric:

• Operating metrics to measure the improved business performance your initiative will deliver.
• Financial metrics to value the improvement and calculate the return on the IT investment.

Operating metrics

These will vary according to the performance improvement initiative. Examples include:

• Sales. Increased online conversion rates.
• Costs. Lower processing cost per application from.
• Productivity. Increased volume of transactions processed per labor hour.

Financial metrics

Once you have an operational improvement, then, you need to put a value on that improvement. If it's an additional sale that should be straightforward (but use contribution, not gross revenue).

Then, you need to measure ROI. Use two measures:

• Calculate the net present value i.e. discount expected net cash flows at a rate that reflects their risk.
• Payback period as a quick-and-dirty indicator of how long your initial investment is hanging out there before it gets paid back.

For the purposes of writing a business case, anybody offering to calculate (or train you) on the following is wasting your time:

1. Simple undiscounted ROI (every project has risk, you can't ignore it)
2. Projecting income statements and balance sheets (accounting concepts, you are making an economic decision)
3. Average rate of return (see comment on 2.)
4. Return on capital employed (see comment on 2.)
5. Return on assets (see comment on 2.)
6. Any kind of monte carlo simulation (you don't have the data and it's analytical madness for 99% of IT projects).