You can reverse a block with 25BTC worth of hashing power. That means each block costs around $6000 to reverse. You can't actually buy enough hashing power to do the reversal, but a rule of thumb would be that you need (value of item / $6000) in confirmations but at least 6 blocks to be very secure. For a "fighter jet" at $20 million, you would need 3-4 thousand blocks so that the value of the hashes securing the transaction was worth more than the transaction.
Is this really a valid way to look at the problem?
Meni Rosenfeld works out how many confirmations you should wait depending on how large the transaction is in addition to the attacker's hash rate here, but I guess this table doesn't address the problem of an attacker temporarily renting hashing power to carry out the attack?
