Solved block = 25 BTC reward * $120/BTC ~ $3,000

An attacker must go back and rework the block that just posted and remove the transaction he's trying to double spend. he must get lucky beating the rest of the network which is ahead and working on a block solution on top of the old block.

Chances are attacker never catches up and any blocks he did find become orphaned (never adopted b/c he's just too slow).  That compute power could have been used to score a real block by building on the valid block.  Cost is 25 btc per fake block you waste time trying to build.  Satoshi figured if you have 10% of the network hash power (which is A LOT) there is a less that 0.1% you could do this if you tried going back 5 blocks.  That is use 10% of the has power to score 6 blocks before the other 90% could score just 1.