A couple of articles by Jakob Nielsen that make the best case for improving website usability - the bottom line, money.
Everyone working in public sector web publishing should read this article. If this doesn't make the case strongly enough, we might as well all pack up and go home...
Do Government Agencies and Non-Profits Get ROI From Usability? - article by Jakob Nielsen
A comparison of usability ROI for a range of websites. Nielsen compares studies done in 2002 and 2008.
His conclusion is that the ROI is falling, but is still substantial (average improvement on average in 2008 was only 83%).
So he estimates that in 30 years, the ROI may not be significant enough to justify usability testing.
Usability ROI Declining, But Still Strong - article by Jakob Nielsen