Pigeon toe, parents always love to hear that they were right. Be contrite and admit you were a rotten kid to not wear your retainers when they put thousands of dollars into your teeth and you didn't even appreciate it! But tell them that you are so much more mature and responsible now, and you appreciate the value of a buck...and you would do everything the Orthodontist said this time (but only say it if it's true!!!). Then ask if they would consider maybe paying for half. And if they do yell at you, don't yell back, just admit they are totally justified and you deserve it.
I look forward to the day that my daughter will say those words to me! I finally just had her braces yanked off (I wanted to do it myself--with pliers!!) because she would not wear retainers or the little rubber bands and was actually damaging the roots of her teeth. Very maddening when you spend thousands which could have gone to vacations or things other family members needed or wanted. Good luck!