Most of the other commenters have focused on whether the business is viable - but it may just be you need to try a different marketing tactic. Paid search is very difficult to turn an ROI on. I ran a search consultancy for awhile and I would tell people that PPC was a lot like blackjack - you can lose a lot of money if you're not paying attention or are new to the game.

The big players like themeforest are going to enjoy higher conversion rates off their paid traffic because (a) they are well-known and (b) they also rank well organically (there have been multiple quantitate studies examining the correlation of high converting paid search campaigns with high organic search rankings).

Try a few other marketing vehicles, get more data, and only then make decisions about what to do next.