For a coin not yet launched the initial financial risk (acquire a lot of coins of a coin with maybe no future value plus building a substantial personal network) seems like a no-go to me. What about an "established" PoS coin? Costs of the way described could be astronomical?!
You can reduce costs by trading in wait time. As long as you have some coins and sit on your wallet and stake them, as time goes by major stakers will "trip over" or get out. You can attack the coin.
Now you might think that it costs a lot to buy a whole coin. But in reality, after the top 300 altcoins, it costs less than 1000 USD to buy one completely:
http://coinmarketcap.com/4You really should not measure the cost to attack a coin in terms of dollars, but should measure it in terms of the percentage of the value of the coin. You also fail to remember that there is nothing that would stop an attack from selling his coins on an exchange after he has attacked the network