An article from the Usability Professionals Association, that's packed with great snippets like this:
"The rule of thumb in many usability-aware organizations is that the cost-benefit ratio for usability is $1:$10-$100. Once a system is in development, correcting a problem costs 10 times as much as fixing the same problem in design. If the system has been released, it costs 100 times as much relative to fixing in design." (Gilb, 1988)I can't think of a home-grown application that couldn't be made more usable. And the thing is, most if not all of them won't be, because the cost of improvements post release is too high.
The ROI of Usability - an article from the Usability Professionals Association
And a suite of resources and case studies from usabilityfirst.com. I've not come across this site before, but this material is worth a look if you were interested in making a case for investing in usability testing.
Usability ROI - case studies and resources