Today for example, 7.4% of bitcoins were transferred. That means $3.3 million worth of BTC. In one year, assuming similar ratios as today (which I've looked through the data and it doesn't seem atypical), that comes to $1.215 billion worth of transactions. Keep in mind a lot these are international transfers which are typically much more costly in classical banking. The amount paid to miners comes to 2% of that, which replaces the cost of all the security that classic banks have to spend on when transferring money, and all of the other transaction costs as well.
Two points. The amount displayed in, e.g., bitcoinwatch is completely bogus. A transaction contains a change. If you transfer $10 to a third party from an account that has $100000, the banking system transfers $10, not $10 + $99990, while Bitcoin does the latter. Wouldn't you be rather pissed off if Paypal charged you 2.9% on $100000 when you moved $10? So the percentage of the cost in Bitcoin is way higher than what you calculated. Secondly, there are so many transactions because they are mostly free. When the fees become a norm, the transaction volume will slow down.
Also, a successful >50% attack would not lead to a complete security breach where all bitcoins can be stolen. It would only open the possibility of double spends by an attacker, so the value of such an attack does not equal the market value of bitcoins. This suggests the risks of lower difficulty/market-cap could be low.
I know that but a lot of people here downplay the consequences of the >50% attack. It is true that there is rather little to gain for the attacker compared to being honest. Therefore, I think we may never see a ">50% attack for profit". But for attackers that only want to make damage (for example a competition to Bitcoin), it can really make havoc. It's the mother of all DoS attacks. A >50% attacker can:
1. Completely halt all confirmations.
2. Reverse all transactions (so halt retroactively) and put them on hold.
3. Annihilate recently mined coins and all transactions where the coins were used (if the chain fork is longer than 120 blocks).
4. Double spend his coins.
And the point that is frequently missed:
5. Allow anybody to double spend. When the chain is forked, all transactions go into a memory pool of the miners. The attacker can be "nice" enough to remove them from his memory pool and allow anybody to connect, submit a new double spend transaction and confirm it. It can allow some non-fraudulent transaction and you will not know what is right and what is wrong.
If you think that it won't crash the BTC value, you are very optimistic. It would create such a mess that it will be very hard to untangle. And do it a few times and nobody will trust the Bitcoin blockchain for anything larger that a few dollar transactions and Bitcoin returns to its amateur status.
Furthermore, operating profit margins right now are quite high for miners. It will decrease significantly once bitcoin inflation slows and the market stabilizes, approaching close to the cost of electricity, which I've seen that you've noted at the moment is $0.75 million a year. This is only about 0.06% of the market value of transactions. There will need to be some profit, so even if we triple that, to 0.2%, that's still very low cost.
As I wrote above, the value of transactions is bogus. The only think non-bogus is the total BTC value (money supply). Relatively to money supply, Bitcoin
is expensive.