Jakob Nielsen has provided one answer to my previous question. By way of summary, I'll offer a recap of the analogies he provides that can serve to convince a client that the processof usability is worth the time and money.

• You wouldn't expect an architect to design the perfect building the first time out, without testing it.
• You wouldn't expect a software programmer to write perfect, bug-free code the first time they tried
• You wouldn't expect a writer to publish something without spending the time to edit it.

All good points, and ones likely to make it into my sphere of reasoning.