This metric is, in essence, equal to 51% of the present value ("PV") of all future revenues derived from bitcoin mining using current Mt. Gox prices. Revenues include both block rewards and transaction fees.
The purpose behind using PV as a measuring tool is to approximate the incentives to miners to build upon the Bitcoin network. The measure can be viewed as an aggregate of all the cost-benefit analyses done by individual miners.
We believe this is superior to other methods of calculating the attack cost, including variables such as current hash rates and current capital costs, because the model is independent of technology advancements. Under the equilibrium model, miners will continue to invest in equipment until they reach the point where marginal cost equals marginal revenue (the point of profit maximization).
some days ago it was 300 mio, now it is 770 mio. is this just because the difficulty rises?
NO, it is because they are using an economic model and the out-of-whack MT.Gox price.
Difficulty does not even factor into it.
They are assuming that miners can instantly respond to the price of Bitcoin. It may be true in 6 months if the glut of ASICs are not profitable; but not now.