It would be cheaper to destroy the network. Go to the places where manufacturer the mining chips (Taiwan etc) and get a great deal to get 51% of the network.
it could be even cheaper to threaten the owners of the 2-3 biggest pools of being thrown out of a window if they do not comply to pwn the network.
So the pools would 51% and fork the network to double-spend a few coins, everyone would shutdown bitcoin payments for a few hours (as happened during the March 2013 fork), and the individuals that make up the affected pools would quickly point their miners elsewhere. p2pool would probably gain a huge share of that.
While it be a severe short/medium term blow to confidence in bitcoin, it wouldn't kill it, and would ultimately make the network stronger since people would be jumping to more distributed mining solutions like p2pool.
I think the pool attack concerns are largely blown out of proportion. Killing bitcoin really requires successfully attacking the crypto.