“The most common user action on a Web site is to flee.”
- Edward Tufte, Information Design Guru

“Usability cost-benefit data shows that including usability in product development actually cuts the time to market and increases sales because usability and ease of use build quality into products and catch many expensive problems early on in the cycle when they can be addressed at lower cost. Finally, working with users from the beginning of a product cycle ensures that the product is being designed so that users will be satisfied.”
- Claire Marie Karat, “A business case approach to usability cost justification.” In, R. Bias and D. Mayhew, Eds. [Cost-Justifying Usability->http://www.amazon.com/exec/obidos/tg/detail/-/0120958112/], Academic Press, NY, 1994.

Just lately, I came across this [nice list of articles-> http://www.rashmisinha.com/useroi.html] published on the ROI of usability by Rashmi Sinha. Here are some excerpts…

“At any instant, millions of people around the world are trying to use or do something that is difficult or confusing. They may be trying to find a product, trying to figure out how a product works, trying to get service for a product, or trying to replace it. Eventually and inevitably, they will begin to lose time and patience. And, no matter what particular answer they are looking for, the question they pose will be the same: “Why would anyone make something this confusing?…the advantages of creating usable products far outweigh the costs. The rule of thumb: every dollar invested in ease of use returns $10 to $100.” from [IBM's Cost Justifying Ease of Use.->http://www-3.ibm.com/ibm/easy/eou_ext.nsf/Publish/23]

“After Dell Computer applied basic usability principles to its e-commerce Web site in the autumn of 1999, its Web sales skyrocketed. Online purchases rose from $1 million per day in September, 1998, to $34 million per day in March, 2000.” [BusinessWeek - Usability is Next to Profitability->http://www.businessweek.com/technology/content/dec2002/tc2002124_2181.htm]