When the first leak occurred a few days ago, the price of a cube was listed at 1.99 btc. That is was around $450, a great price for 30Gh/s.
Today 1.99 btc is $622. You are getting close to what you need to pay for a 30 Gh/s BFL box on ebay. By this weekend, if btc keeps rising, it may be hopelessly overpriced.
There's some truth to this... but it still mostly makes sense to price mining equipment in btc terms regardless of the exchange rate.