Yes, Code Canyon "seems" to suck. But there is a catch for everyone. There are coders who tried with a script and two and hadn't found the ROI reasonable, so they left; there are coders that worked on it (whatever the costs) to create a side income and there are coders that worked on it to showcase their skills, like me.
It worked out. I won bids with Code Canyon and also raised significantly my rates. So Code Canyon paid out for me, if though Envato didn't pay me a check.
Back to your calculation, you are completely wrong. Code Canyon is not a freelance website. Code Canyon is a market. You build a product and you wait. Envato brings the customers, but they won't buy anything, they'll buy only what they need. Not all ThemeForest sells like mad, but a few ones sells for more than $30K.
So when you are an Entrepreneur, you need to be the Coder, the Sales Man, the accountant... all of them. You must study the market yourself. That's one of your jobs and Envato just makes things easier for you, for 0$. (Yes 0$; bring customers yourself and you'll get 30% for it, then snatch 50%-70%, so Envato takes the big money when you don't bring the customer).
Plan, Work, Execute, Market your product and hope for the best.