Drupal is open source. That means the software is free, there's no license cost.
The Drupal source code is freely available, so if you need it changed, it's easy and cheap to do.
Companies like Themesandmodules.com make money by providing programming and support services around the free software.
Using open source Drupal has many advantages.
The marketplace for Drupal services is huge, and therefore very competitive.
Not only do you save money on license fees, but you'll also tend to save money on services. There are thousands of Drupal companies to choose from, so you are secured against expensive vendor lock-in problems endemic in closed-source products.
Closed source Website Content Management Systems (WCMS), on the other hand, have license fees ranging in price from a few hundred dollars to hundreds of thousands of dollars per website.
Only the company selling the WCMS, and at best a small number of partners, have the expertise customize it and support your business. The market is smaller, less competitive and prone to huge risk from vendor lock-in.
Using open source Drupal means you have the freedom to choose.
Using Drupal puts the customer, not the vendor, in the driving seat.
Using Drupal means you get more powerful solutions at a lower cost.
