Not a cop, but As a practical matter my understanding is that they wait until money is exchanged for this reason. Paying the money is an overt act which is proof of intent and that an agreement between two parties is made. Otherwise “I was just blowing off steam, it was just two guys talking” is a legitimate defense. In practice people must have lived in a time before conspiracy to commit murder was a crime itself and they didn’t like the outcomes (breaking up a plan to murder before it happens and they get away with no punishment) so it becomes a law.