I think that both Todd and Morten have raised some very good points about difficulties with the theming landscape in the Drupal world, but I would like to offer my take on the issue.
First I will put on my economical hat and later on I will add some points from a developer point of view.
Product Life Cycle:
In the Joomla templates and Wordpress themes business, theme developers have already gone through all the stages that us Drupal folks will have to face in the near future. Premium Theme developers in Wordpress and Joomla appear to have gone through the classical Growth-Slump-Maturity pattern: