[hn_throwaway_99]

The issue with saying how much a 51% attack "costs" is that ALL cryptocurrencies (and, really, ANY currency) is just based on the trust that the currency has "value", meaning that it would be accepted as payment for real goods and services.

A 51% attack on bitcoin would be easily noticeable. If a 51% attack was really "viable", it means that essentially bitcoin would have $0 value, because all of its value is based on the trust that the blockchain is real and verified. The community at large would essentially just ignore the rogue chain, or rather probably boost other existing resources to back it out.

[meowface]

How would the community collectively agree to ignore the rogue chain in a timely fashion? If a client announcement is sent out, that'd help, but there'd still be a lot of people stuck on the original chain (for example, some people on vacation or something). For at least a few days, there'd probably still be a lot of trust to capitalize on, and they could possibly get away with a few big heists (e.g. making a big purchase with BTC and then reversing the transaction after you've received the items).

Also, if the community does ignore the rogue chain, what's to stop the attackers from switching their attack to the other non-rogue chains as soon as they seem to gain traction? If they can hypothetically reliably sustain 51%+ power for weeks, they could potentially perform a DoS on an entire currency. And you're right, Bitcoin would probably soon reach close to $0 (though would probably slowly bounce back once/if the attack seems to be permanently thwarted).

If I'm understanding your last sentence right ("probably boost other existing resources to back it out"), then I also think that'd be the most likely scenario. A serious sustained 51% attack (lasting beyond several days) would turn into a Dragonball Z-like battle, with both sides firing continuous energy beams at each other in parallel. Each side would be trying to increase the magnitude of their beams to keep the other side's beam from collapsing theirs. I think the legitimate mining team would have a lot more energy in reserve (e.g. good samaritans who start mining for the first time to help push away the attackers - kind of like Goku's Spirit Bomb, which Frieza can't replicate) and would probably win. However, if for some reason the attackers can win for weeks at a time, then it'd be a serious DoS.

Disclaimer: I might be misunderstanding something important here. Not a cryptocurrency expert whatsoever.

[WAHa_06x36]

So: Short a large amount of bitcoins, 51% attack until value plunges, earn massive amounts of money.

Of course, the problem with this is that there is no exchange that you can trust enough to actually do this.