It's almost like if it were true that if it only cost $400/hr to 51% attack the BTC chain, we would actually see it happening much more often.
If we cut the hash rate tenfold, which is definitely significant, you'd still need to find someone who has hundreds of thousands of dollars of motivation to perform an attack. Having that motivation is hard when it takes a huge amount of double spending all at once, the attack will damage the integrity of the coin, and clients can be modified to be highly resistant to it.