Obviously the big advantages of a paid theme, from a reliable seller, are updates and support. Support being the biggest. Not saying you can't get support with free theme's, but it's often hit and miss. Usually your on your own. Same thing for free v.s. paid plugins.
But on the other side, you can buy a theme that looks like it's going to work for you, only to find out it has some issues.
My last big conversion for a site from an old shopping cart to wordpress, I went through a ton of paid theme's (tried them before i bought), only to ditch one after another. Usually just little things, but enough to make it a deal breaker. So in that instance, I could have wasted alot of cash. In the end a free theme that was the best for my situation.
Sorry, that is not a very good answer. Best I can say from my experience.