Side air bags shouldn't be an option on Mustangs; they should be standard. I don't understand why anyone would not get them, unless they intended on replacing the seats like the person above.
Just $300 to save the life of your best friend, wife, child, sibling, etc... Can you imagine not getting the side air bags, killing someone you love when you're smoked in the side, and having to live with the fact that they may have been saved if you weren't such as cheap ass?
My parents bought an Escape a couple years ago, and I told them to get the side air bags. Well, the car salesman claimed how less than 3% of Escapes had them, and that none of their Escapes were ordered with them. He said if we did want them, they'd have to order the vehicle and it'd take 3 months. The exact Escape we wanted was on their lot, minus the side air bags. I demanded my parents wait and order the car, and they did.
About a week after it was delivered, there were side impact crash tests on Dateline on all small SUV's. All of the vehicles in this class, including the Escape, were rated "poor," claiming that the passengers were likely to die. At the end of the story, it said, "But Ford insisted that we test one of their Escapes with the side air bags, and they provided us with one...... With the side air bags, the Ford Escape took our highest rating in the side impact test, "excellent," with no signs of head injury."
If you want to be cheap, skip the Shaker 1000, not the safety devices.