To temporarily disable the startup sound, just press the “Mute” button on your keyboard (that’s the F10 key on a MacBook) before shutting down or restarting your Mac.
To permanently mute the startup sound, open the terminal and type "sudo nvram SystemAudioVolume=%80" (or sudo nvram SystemAudioVolume=%01, or sudo nvram SystemAudioVolume=%00 or sudo nvram SystemAudioVolume=" " depending on some models).
To restore the startup chime, type: "sudo nvram -d SystemAudioVolume"
There, I just saved you $11.