Drupal is GPL, so a module written specifically for Drupal must always be redistributable under GPL as well since it's a derived work. That places a natural lim

Drupal is GPL, so a module written specifically for Drupal must always be redistributable under GPL as well since it's a derived work. That places a natural limit on the feasibility of module purchases, since anyone could buy it and then mirror it for free. The correct answer here is that if you don't like Drupal's free & community-focused development model, you're likely better off switching to some off-the-shelf software. Alternatively, you can pay a consulting company to maintain the module, which is how many of them get written anyway (and then given away for free). So if you focus on modules with commercial backing, you've got a pretty close approximation without giving up GPL freedoms.