Breaking savings out "by module" is very specific to each company, industry, spend type, level of automation, etc. What we've seen from analysts like Aberdeen, Hackett, IDC and AMR are "averages" that are in the 5% - 15% range for moving from a peer level of functionality to best in class. If you're not even at a "peer" level, your savings can easily exceed these ranges.
All of the savings cannot be captured with a single solution, and in some cases, interdependencies are critical to the net benefit (for example, sourcing can identify savings, but without compliance drivers of P2P, much of the savings may not make it to the bottom line).
Several approaches can be taken to refine savings estimates include:
a) Do a "spot audit" of areas that are particularly concerning
b) Ask for assistance with looking at industry standards for similar businesses (Ariba can help with this)
c) Focus on a particular solution area to start based on your company's realistic ability to change and ensure compatibility with future expansions