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The attack sould last 1h, spending those 400 BTC for 80 times instead of just 1, paying attention to pay 80 different not-so-sofisticated providers as mentioned above. At the current value of 1 BTC = 0.40 FRN("USD"), this would mean buying 32,000 FRN ("USD) worth of goods and services. On the other hand the cost of compromising the Bitcoin network for 1h with 4000 malicious CPUs would be, at a current price of FRN("USD") 2.10/CPU/hour (Amazon Quadruple Extra Large Cluster Compute Instances), 8,400 FRN("USD"), plus the initial 160 FRN("USD"), for a total of FRN("USD") 8,560. Considering harvesting FRN("USD") 32,000 worth of goods and services that's a pretty good +220% ROI in just 1h. Pretty good compared to your average investment.

Your calculations are garbage. You cannot spend coins 80 times in an hour. The attacker has the power to rewrite a history that doesn't include him spending the coins, that is all. He can't simultaneously convince 80 people that they have the same coins. In the slow shipping example you need to let the shipper think he has coins until after he ships, then you can pull them back. You can't do this 80 times in 2days. You would need about 40 days if people are shipping same day.

If you try to spend them twice in an hour at, say, MtGox you won't ever get credit and can't get dollars because he waits for 6 confirmations. If you go for 2 hours you can spend them there twice this will not get you double your money because you will be bidding the price down by buying quickly which you will have to do since your cover is blown when you stop paying $8560/hr.

Not to mention that Mtgox (the only site with anywhere near enough bids to get your 'investment' back) has some max withdrawal per day.

Once again, the reason this is not profitable is that you have to match the entire network, but you only get a little tiny slice of the flow, not "everything conceivably for sale for bitcoins"

And this attack does not get more profitable as USD/BTC increases. Difficulty has been increasing faster than price for a long time. It is getting more costly at a faster rate than the payout is growing.