Accessible, standards-compliant, quality sites should cost more than their shabbily constructed brethren. You may worry that you’ll lose business to your competition, but realize that you’re not really competing with those who produce inferior work.
Mike provides a great breakdown of the importance of Accessibility from a business perspective. If you build accessibly, you are by definition better at your job as a Web professional than a person who does not. The site you provide your client is of a higher quality, which will provide them benefits beyond doing the right thing for all visitors. I won’t go into all of the details as I hope to write an article around the business benefits of accessibility some time soon, but check out the article and Mike’s blog which provide a wealth of information. Mike is a great voice in the Web Development and Accessibility world, and well worth adding to your RSS feeds.