How do you get $300 million?  Even at $100 per GH/s, you could buy the whole network for $50 Million.  This is a drop in the bucket for any government.  If they pull it off, no more transactions are processed, everyone stops mining and the incentive to mine drops to zero.  

The only meaningful value is how much the government needs to spend to destroy the network.  Right now, that's no more than $50 million and that's being very generous.

What am I missing?