WordPress theme marketplaces where you can buy commercial WordPress themes are rapidly gaining popularity, and must admit that I have bought and used some commercial themes myself. Unfortunately many people are under the impression that these cheap themes are the perfect solution for all their needs. While a $30 might seem good value, this is not necessary the case.
At WordCamp Chicago 2011 Andy Stratton gave a presentation called “Diet Pills, SEO, and Theme Frameworks”. He talked about WordPress themes, frameworks, marketplaces and a lot of related misconceptions, and some of the many problems with commercial themes including SEO, customization, child themes.
So while there are some cases where a commercial $30 theme could work, usually these themes are slow, bloated with unnecessary options and will require a lot of expensive customization.