25 BTC per block * $127.8 @ mtgox = $3195

An attempt to double-spend a one-confirmation block is an attempt to:

1. Identify a "sucker" that trusts 1 confirmation transactions,
2. Buy something unreversible (rare) that you receive in seconds or minutes,
3. Your transaction is included in a network block, you have one confirmation,
4. You need to independently create two blocks before the rest of the network creates one, or create three blocks before the rest of the network creates two, to replace the one-confirmation block with one of your own that sends the coins elsewhere.

The probability of being able to pull all these off is very low. #4 requires you to mine blocks that nobody else accepts if you fail, blocks that could have earned you 25 BTC.

You also must be able to afford many losing attempts to double spend - if you were buying something for 1000 BTC, you would be spending 1000 BTC multiple times before one double-spend attempt was able to refund you one of the 1000BTCs.

Satoshidice accepted 0 confirmations transactions which require 0 miner expense, had an instant reward payment, attackers only needed a 2% success rate to profit, and yet it was still difficult to pull off an attack.