If you head over to ThemeForest.net you're going to pay around $50 for a single use, heavily restricted (use once only) Theme. or Thousands of dollars for the extended license option.
But here's some really good news for you, my good friend Matt Garrett has just opened WP Toolkit - Lifetime Developers Edition
It's filled with 380 Premium WP Themes and 250 Premium WP Plugins
It's a massive library of "ThemeForest Quality" WordPress Tools with hundreds of the world’s best WordPress plugins and themes all in one incredible package, and all offered at literally ‘pennies on the dollar’, complete with Developers rights... (Install on unlimited clients sites)
I've also prepared incredible Brand New video background with reseller licenses as bonuses for you, plus others.
Best Regards,
Mark Mason