One of the problems of using free themes is that your website won’t be unique. There could be hundreds of other webmasters using that same WordPress template. Think about it, if you come across a free template that you think looks great chances are that so did many others.
Another problem with free WordPress templates are that they usually come with many restrictions. One requirement that the free WordPress themes usually have is that you have to link back to the website that you got it from. , if you are trying to build the ranking of your website for SEO purposes you may not want to give away a link. Another problem can be that you will not be permitted to use the free WordPress template on a commercial website.
For the most part when you pay for a WordPress SEO theme you will be getting a better quality. Large sites that sell WordPress SEO templates hire designers to create professional SEO WordPress templates, and then they have the right to resell the WordPress SEO theme.