How much money is a double spend worth? I keep hearing people say that 0 confirmations are safe at small dollar amounts because it would take more money to do a double spend hack. At what value does it start to become unsafe to trust a 0 confirmation transaction?
The easiest way to do a double spend against someone who accepts zero-confirmation transactions is with a Finney attack. To perform a Finney attack, you must first mine a block and be willing to risk losing that block. So the answer is that if the value of the transaction is small relative to the risk of losing a block reward, you don't have to worry about the attack.

Using today's numbers: An attack with a one minute window has about a 10% chance of costing you a block. The block reward is 25 Bitcoins and a Bitcoin is around $600. So for amounts much less than $1,500, you don't need to worry about a double spend. Nobody's going to risk losing $15,000 to steal $1,500.

Note that this assumes you know how to accept zero confirmation transactions! You must watch at multiple points in the network and ensure the fee is adequate.