Try themeforest they have good framework and excellent theme. The pros and cons of using free vs. paid theme is that in free u might
1. not get updates
2. Could be bad coded (there r many out there, use wordpress site to download free themes only)
3. You can make free child theme as they r open source.
Whereas paid themes
1. Get update frequently and u will get it too
2. coded good from SEO and cross broswer point of view.
3. might not have license to create free child themes (majority don't let u unless u buy a extra license)
Every client wants to have minimal budget and yet a good theme, it also depend of client some are noob, meaning they don't wanna know if free stuff is good or not or search it out to get what they want. so they buy it. if one is a good designer and developer. he can develop a paid sort of theme on its own. besides client and people pay extra bucks because they design is more good than freeonces.
I hope it helps