I never did the bun thing, my kids hate hamburgers, but I remember reading about it before my trip last year and thinking it was very clever. I cut coupons and save pennies all year so that when I go to Disney I don't have to worry about it. If this is what worked best for my family, I would just buy the bun for 1.99 and not worry about it.
I live in NJ near 6 Flags Great Adventure, and they don't let you bring ANYTHING into the parks. I once had a ziplock baggie of teddy grams in my purse for my 2 YO, and the security guard threw them in the garbage before he would let me in. I guess I think it's great that Disney lets you bring in your own food and water at all. I saved a lot last year on carrying in my own water, so I didn't stress too much over food prices. And by the way, the food at 6 flags is horrible and twice as much as the very good food at Disney.