The cost of redesigning an old table/image based site around CSS can be outweighed by the bandwidth savings as associate art director for ESPN.com Mike Davidson points out:
http://www.mikeindustries.com/blog/archive/2003/06/espn-interview
In the interview he explains the logic behind the ESPN.com redesign and the massive bandwidth savings they were able to achieve (approximately 730TB a year). Given the actual saving per hit was only 50Kb the huge overall bandwidth illustrates the magnitude of the Web space. Designing high-bandwidth sites with lots of images, table-based design or even worse Flash is just asking for trouble in the long-term if the ultimate objective is to create a popular site. It is no good generating lots of advertising revenue from page views if this traffic ends up costing far more.