The important questions for everyone else are:
How much money does the median author/designer on ThemeForest earn?
How much material does an author/designer need to upload in order to get that median income?
To be honest, I wouldn't be interested in the median author/designer. They're most likely putting little to no effort in to promoting their work. Discovering the level of interest you need to create to get near the top earners would be interesting.
Orman doesn't have very many themes on the site, but he does do a lot of off-site promotion on his blog (and perhaps elsewhere) and give away a lot of material as part of that. That could be a huge time-cost, but may be well worth it from his point of view given the earnings.
I may not have read it closely enough, but I couldn't work out how much of his earnings came from selling his own themes on the site, and how much came from the affiliate income he earned from selling other people's themes when the entry point to the site has included his affiliate code. That affiliate income could really add up if he's sending plenty of traffic their way.
This question comes up all the time on the Themeforest forums, but I haven't seen a satisfying answer. Most authors seem to think that if you have to do significant outside promotion, it isn't worth giving 30-50% of your sales. At that point, you might have better luck selling direct, like Thesis.
Whether that's worth the 30% you're giving away, I don't know. I do know it means the designer/author hasn't had to program (or pay to have programmed) any payment gateway, doesn't have to deal with refunds, isn't a higher target for being hacked as they're dealing with e-commerce, etc etc.
Basically, if you have a small number of themes and don't like programming, my gut feeling is you're better off on a large-ish site who is getting traffic independently from your efforts. If you can make a ton of good themes and handle all the programming and marketing, being on your own starts to make more sense.
Er, here we go, the answer seems to be "it depends": http://themeforest.net/wiki/selling/getting-paid/payment-rat...
He is in the markets for less than one year, and has 1000-1999 referrals. As a CodeCanyon member, most signed referrals deposit money. Let's assume the average deposit per user is $30 for Orman. He gets $9 per user.
1999 / 12 = around 150 user per month. 150 * 9 = $1.35k
Not much compared to the actual sales, so most of his sales comes from traffic ThemeForest brings.