Check out The ROI of user documentation: you could break even if you avoided 3 support calls per week by Ellis Pratt. Here is an excerpt:
We’ve been experimenting with another spreadsheet for calculating the Return on Investment (ROI) on User Assistance. We wanted to look at: how many support calls an organisation needs to have resolved by users reading the Help content instead of calling Support, before it starts to see a return…
The full article is available here.