If you are setting up a WordPress blog and are looking for free wordpress themes, there are many. But there are also some advantages to purchasing a premium wordpress theme. And the cost is usually very reasonable.
WordPress Hack has a good article about WordPress Themes and choosing the right wordpress theme for your project.
They cover issues around support and customization of the theme.
One interesting thing I learned about WordPress Themes from the article is that there is a problem with people selling bad themes as premium themes. Since I am very careful with my shopping, I have never experienced that. But I can see how someone new to WordPress could easily be fooled.]
The good thing is that you can find and install WordPress themes in a few minutes, and if you do not like them you can change the theme in seconds. It is very nice. Go try a few free wordpress themes first, and if the basics are not enough for you, then go shopping for a premium WordPress Theme.