For the network to be completely secure, theoretically, it would need >50% of the world's computing power mining it.
and theoretically it costs many millions to build a specially-designed zero-gravity, nitrogen-pressurized pen for use by astronauts. when in reality a twenty-five cent lead pencil does the same job.
bitcoin doesn't need to prevent every attack vector. because attaining 51% provides so little benefit to the attacker relative to the cost, there isn't a huge level of investment necessary. the reason bitcoin mining has turned into an industry is because the mining reward subsidy is fixed and just happens to support a lot of investment. bitcoin doesn't need twenty-three thash/s to be secure from attacks, but with $3 million a month of bitcoins issued, that is how big it grew.