I just typed in "portfolio" onto ThemeForest and I have to say, pretty stiff competition. Many of the themes I looked at are VERY high end, clearly created by some kind of design professional.
Definitely not an area your average web-developer can just drop into. I can definitely write the code but you also need a good artistic/design mind. Just to make everything more coherent and to create a real theme that runs through the template.
Not only that but ThemeForest users now demand support for their investment. It mostly means keeping up with the rapidly updating Wordpress framework but most of the highest rated themes have some sort of post-purchase support. I think that is why most of the coders are from areas where the cost of living is lower (Eastern Europe/Asia).
Definitely not an area your average web-developer can just drop into. I can definitely write the code but you also need a good artistic/design mind. Just to make everything more coherent and to create a real theme that runs through the template.