It's a little over what you'd expect to pay for a normal theme but as someone who does this, albeit somewhere else, I can confirm, knowing what Joomlashack offer, it's well worth the money - especially if you are serious about working and developing Joomla.

You get to use as many of the themes on as many domains as you want, you get access to a developer specific forum at Joomlashack - which will contain a host of good stuff and you get access to the JS Joomla University.

The developer fee is $499 a year for everything above and more - read about it over at Joomlashack.