Building on Drupal to Reduce Costs
Building upon Drupal because it's 'cheaper' is a very poor reason to build on Drupal. It may be cheaper in the end than hacking up some closed-source commercial CMS to include the functionality that Drupal provides in its core distribution. But there's at least one thing to keep in mind:
Drupal talent is expensive. Right now there's a shortage of talent, and in any market where the supply is low and demand is high, the price will go up. There are no shortages of examples within the industry: Perl programmers spring to mind, compared to programmers fluent in more common languages like Java or Visual Basic.
The reasons to build upon Drupal are the power it gives developers, the freedom it offers you, and the potential for the future. I do wish people wouldn't kid themselves by thinking that it's inferior to commercial offerings because you pay for the commercial software.
For some commercial software you will pay. PHPFox -- commercial web-based social networking software written in php -- has a license fee of $300 per year plus an extortion fee of $75 to permit you to remove the 'Powered by PHPFox' credit at the bottom.
The advantage of using Drupal is that you can put in sweat equity towards building your site: doing so can considerably lower your budget (and teach you neat things about Drupal). Commercial offerings have no such ability.